Laser mouse support

[vilhalmer]

I've been working on supporting VROverlayInputMethod::Mouse by forwarding the events through to Input.parse_input_event, and it's working. In the process I've discovered that the builtin mouse mode is largely useless. It captures input globally, meaning your other hand stops working within the scene application, and also can no longer interact with any other overlay applications. It also overrides all controller bindings and sends left/middle/right button presses as if it were a real mouse, using bindings that are shipped by Valve with the VRCompositor. All configured actionsets stop receiving events.

Given these limitations, I have ended up reimplementing my own mouse system using raycasting with the provided ComputeOverlayIntersection API. Based on the behavior I've seen in utilities I have installed, this seems to be what nearly every other overlay application on Steam does as well. The downsides of this approach are a) the pose used is not standardized and thus different overlays feel different, and b) the laser itself is not visible in-headset.

This leads me to the following question. Do we:

• Provide just the official mouse mode, leaving it to developers (or something in assetlib) to override if desired
• Provide just a custom mouse implementation, and ignore VROverlayInputMethod entirely
• Both, perhaps with the custom mouse asset as part of the demo

Currently I've implemented my custom mouse system by just emitting the same events that openvr does, which means the same handling code intercepts them and converts them to godot events. The change is therefore more or less drop-in.

I don't feel strongly in any particular direction, and would be happy to open source my custom mouse implementation under the Liminetics namespace if we don't want to take a stance on it in this project. There is definitely a strong argument for keeping these bindings behaving as close to OpenVR proper as possible.

[beniwtv]

I feel like we should do both; I can see use-cases where that might make sense in different scenarios.