This change adds support for running XR projects built with the `d3d12`
rendering backend. The XR backend hooks into the setup for the D3D12
render context in order to use the desired device and command queue for
submission to OpenXR. The XR backend takes care of importing the D3D12
swapchain images into the render context.
As part of this process, three issues are addressed:
- Ensuring that resource state transitions are only done on textures
that require them.
- Enabling view instancing in the PSOs for multiview render passes.
- Addressing a bug in the D3D12 runtime where PSO creation may fail
when front face detection is used.
Please refer to #86283 for additional discussions on the implementation
details.
Not everything is yet implemented, either for Godot or personal
limitations (I don't have all hardware in the world). A brief list of
the most important issues follows:
- Single-window only: the `DisplayServer` API doesn't expose enough
information for properly creating XDG shell windows.
- Very dumb rendering loop: this is very complicated, just know that
the low consumption mode is forced to 2000 Hz and some clever hacks are
in place to overcome a specific Wayland limitation. This will be
improved to the extent possible both downstream and upstream.
- Features to implement yet: IME, touch input, native file dialog,
drawing tablet (commented out due to a refactor), screen recording.
- Mouse passthrough can't be implement through a poly API, we need a
rect-based one.
- The cursor doesn't yet support fractional scaling.
- Auto scale is rounded up when using fractional scaling as we don't
have a per-window scale query API (basically we need
`DisplayServer::window_get_scale`).
- Building with `x11=no wayland=yes opengl=yes openxr=yes` fails.
This also adds a new project property and editor setting for selecting the
default DisplayServer to start, to allow this backend to start first in
exported projects (X11 is still the default for now). The editor setting
always overrides the project setting.
Special thanks to Drew Devault, toger5, Sebastian Krzyszkowiak, Leandro
Benedet Garcia, Subhransu, Yury Zhuravlev and Mara Huldra.
