Set up OpenXR display

[marceline-cramer]

Dependent on #19

There are several steps that need to be figured out as we go along:

• link OpenXR loader library
• set up OpenXR system and session
• initialize in cli/main.c
• initialize camera and viewports
• wait, begin, submit, and end frames

[MilesLitteral]

Camera is initialized in cli/main.c?

[marceline-cramer]

No, openxr_display is initialized, and the display creates the camera, which is retrieved by cli/main.c to get the initial render pass to feed to the renderer.

This is temporary, in the future, the renderer will dynamically consume displays at frame time instead of initialization time.