feat(renderer): add iSurfaceOffset and iWindowResolution shader uniforms

[yabbal]

Summary

Custom shaders currently have no way to know where their surface sits within the window. Each split gets its own coordinate space starting at (0,0) with iResolution set to its own dimensions. This makes it impossible to create effects that span the full window (gradients, vignettes, CRT curvature) — they get duplicated per split instead.

This PR adds two new Shadertoy uniforms:

• iSurfaceOffset (vec4) — .xy = pixel position of this surface's top-left corner within the window
• iWindowResolution (vec4) — .xyz = total window size in pixels (same convention as iResolution)

With these, shaders can compute window-global UVs:

vec2 globalUV = (fragCoord.xy + iSurfaceOffset.xy) / iWindowResolution.xy;

Changes

• Shader interface: 2 new uniforms in shadertoy.zig + shadertoy_prefix.glsl
• Renderer: new surface_offset / window_size fields, populated per-frame, with safe defaults (offset=0, window=surface size)
• Message system: new surface_position message for thread-safe updates
• Core Surface: new surfacePositionCallback for apprts to report position, with crash metadata
• macOS (Swift): updateSurfacePosition() in SurfaceView_AppKit.swift — computes position via convert(bounds, to: nil), flips Y axis, converts to backing pixels. Called on every sizeDidChange and viewDidChangeBackingProperties
• GTK: computes position via computeBounds relative to the root widget, scaled by getScaleFactor() to physical pixels. Called on every resize

Backward compatibility

Fully backward-compatible. Defaults (offset=0,0, window_resolution=iResolution) mean existing shaders behave identically. No existing code paths are modified.

Related

• Closes the shader aspect of Feature Request: Unified background image across splits #10053 (unified background across splits)
• Enables window-spanning effects for the custom shader ecosystem (gradients, vignettes, unified CRT, etc.)

AI disclosure

I used Claude Code (Anthropic) to assist with developing this PR. I've reviewed and understand all the code.

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Reopened from #11981 (original was closed while fork was deleted).