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