Addresses T77127 (Controller Drawing).
Adds VR controller visualization and custom drawing via draw
handlers. Add-ons can draw to the XR surface (headset display) and
mirror window by adding a View3D draw handler of region type 'XR' and
draw type 'POST_VIEW'. Controller drawing and custom overlays can be
toggled individually as XR session options, which will be added in a
future update to the VR Scene Inspection add-on.
For the actual drawing, the OpenXR XR_MSFT_controller_model extension
is used to load a glTF model provided by the XR runtime. The model's
vertex data is then used to create a GPUBatch in the XR session
state. Finally, this batch is drawn via the XR surface draw handler
mentioned above.
For runtimes that do not support the controller model extension, a
a simple fallback shape (sphere) is drawn instead.
Reviewed By: Severin, fclem
Differential Revision: https://developer.blender.org/D10948
9dda65455b