Build system improvements: OTIO vcpkg overlay migration, general fixes, cross-platform docs

[OlafRocket]

Pull Request TITLE

[ Build system improvements: OTIO vcpkg overlay migration, general fixes, cross-platform docs ]

Summarize your change.

• Migrated OpenTimelineIO from a git submodule to a vcpkg overlay port.
• Added Ninja generator presets for macOS and Linux (Release, RelWithDebInfo, Debug).
• Added aqtinstall as an alternative command-line method to install Qt.
• Restructured the Windows build documentation around a unified Developer Shell setup, with separate sections for the Visual Studio and Ninja generators.
• Several Windows build fixes: corrected QML DLL output directory, fixed Python install path mismatch, replaced install(CODE) with install(SCRIPT) in scripts/qt_install so packaging works.
• Worked around Qt 6.5/6.8 FindWrapOpenGL.cmake AGL bug on macOS.
• On Linux, cleaned up obsolete submodule/vcpkg-pin/preset-edit steps, replaced them with a flexible Qt6_DIR setup section, added an optional Ninja build path.

Describe the reason for the change.

Simplify the build system across all platforms by minimising manual setup steps. The goal is to handle all source code dependencies through vcpkg, while build tools (CMake, Ninja, compiler) are installed via each platform's native package manager or from open-source releases. Added an alternative way to install Qt from the command line that doesn't require a Qt account.

Describe what you have tested and on which operating system.

Built and ran on a clean system on:

• Linux Rocky 9.7
• Windows 10
• macOS Sequoia 15.7

Add a list of changes, and note any that might need special attention during the review.

Core build system:

• Migrate OpenTimelineIO from git submodule to vcpkg overlay port (cmake/vcpkg_overlay_ports/opentimelineio/). Removes extern/otio, .gitmodules, cmake/otio_patch.diff, and the OTIO_SUBMODULE option.
• Add VCPKG_OVERLAY_PORTS path so the new OTIO overlay is picked up.
• Pin python3 to 3.11 in vcpkg.json to align with VFX Reference Platform 2024.
• Lowered CMake minimum from 3.28 to 3.26 (default on Rocky Linux 9).
• Remove hardcoded Qt6_DIR from the default preset. Users now supply it via the Qt6_DIR environment variable, a CMakeUserPresets.json, or a one-off -D flag (documented in each build guide).
• Add Ninja generator presets for macOS (arm / intel) and Linux — Release, RelWithDebInfo and Debug variants. Aligns with existing Windows Ninja presets.

Windows related changes:

• OTIO portfile: on Windows, move .dll files from lib/ to bin/ (vcpkg convention for applocal.ps1 discovery) and rewrite generated Targets-*.cmake files so IMPORTED_LOCATION resolves correctly.
• Fix scripts/qt_install to use install(SCRIPT) + configure_file instead of install(CODE), so ${CMAKE_INSTALL_PREFIX} resolves at install time. Previously the configure-time prefix was baked in, which broke the CPack package target because CPack uses a different (staging) prefix.
• Remove the redundant early windeployqt call in src/launch/xstudio/src. The late call in scripts/qt_install now covers everything and runs after all other install rules have populated the install tree.
• In cmake/macros.cmake (default_options_qt), add RUNTIME_OUTPUT_DIRECTORY alongside LIBRARY_OUTPUT_DIRECTORY so *_qml.dll files land in build/bin on Windows (Windows uses RUNTIME for .dll, not LIBRARY).
• Fix Windows python install path mismatch in python/CMakeLists.txt: the build produces lib/python3.11/site-packages (Unix layout, because vcpkg's Python uses that layout on Windows), not Lib/site-packages.
• Generate build/run_xstudio.bat — a dev launcher that puts Qt's bin directory on PATH and forwards arguments to xstudio.exe, so xstudio can be run straight from the build tree without a windeployqt pass.
• Un-hide WinDebug preset.

macOS related changes

• Replaces the previous manual-edit workaround Qt 6.5/6.8 bug in FindWrapOpenGL.cmake (which links -framework AGL, a library long removed from the macOS SDK) by pre-creating the WrapOpenGL::WrapOpenGL target so Qt's find-module returns early. Qt bug fixed upstream in Qt 6.9+; this workaround can be removed once the minimum supported Qt is 6.9.
• Added a "Tell CMake where Qt is installed" section covering Qt6_DIR env var, CMakeUserPresets.json, and -D override.
• Added an optional "Faster builds with Ninja" section with brew install ninja and the new presets.

Linux related changes

• Adds VCPKG_OVERLAY_TRIPLETS to the linux-base preset so xstudio's overlay triplets (x64-xstudio-linux) are picked up automatically.
• Lowered CMake minimum from 3.28 to 3.26 to match the default version shipped on Rocky Linux 9 — broadens distro compatibility without needing a manual CMake upgrade.
• Added Ninja generator presets for Linux: LinuxNinjaRelease, LinuxNinjaRelWithDebInfo, LinuxNinjaDebug.

Documentation

• downloading_qt.md: add aqtinstall instructions (command-line, no Qt account required) as a recommended alternative to the GUI installer. Covers Linux, macOS and Windows.
• index.rst: add "Choosing a guide" section explaining the split between vcpkg-based guides (recommended) and distro-native guides (advanced).
• linux_generic.md: restructured around the new Qt6_DIR / user-preset workflow, removed submodule instructions, added optional Ninja section.
• macos.md: minor updates to reflect current dependency list and Qt workaround now handled in CMakeLists.txt.
• windows.md: full rewrite, unified "Set up the build environment" section using Enter-VsDevShell with -SkipAutomaticLocation, separate build sections for Visual Studio and Ninja generators, corrected Qt path example (C:/Qt not C:/Qt6), add user-preset examples.

[linux-foundation-easycla]

• ❌ - login: @OlafRocket / name: Olaf Razzoli . The commit (21debc3) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.
• ❌ - login: @OlafRocket / name: OlafRocket . The commit (8ab7f45) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.