Submit Depth Buffer to OpenXR

[bjornbytes]

• Enable XR_KHR_composition_layer_depth
• Create a second swapchain for depth textures
• Probably need to start using xrEnumerateSwapchainFormats to see if depth formats are supported
• Question: what if depth swapchain and color swapchain lengths don't match? Fail?
• Need a way to set the depth buffer of a Canvas (at creation time), maybe .depth.handle entry in CanvasFlags
• Each Canvas gets one of the depth swapchain images
• Can always fall back to old lovr-managed depth texture (extension not supported, format not supported, etc.)
• Add XrCompositionLayerDepthInfoKHR struct to XrCompositionLayerProjectionView with depth info
• Plug state.clipNear and state.clipFar into the nearZ/farZ fields.

[bjornbytes]

Started working on openxr-depth, very WIP/incomplete but uncovers some issues:

• Since the order that swapchain images are given to you is completely arbitrary, we can't guarantee that color/depth indices will match for a frame. This makes it impossible to precreate Canvases with color/depth textures, since we don't know which ones will be used together. The solution is to just create a Canvas for each color image, then create native Textures for each depth image, then dynamically attach whatever depth texture OpenXR gives us to the current color Canvas every frame.
  • This also lets us tolerate different numbers of depth and color images.
• Other than that there's just a lot of reorganization around having 2 of everything -- swapchains, image counts/indices, etc.

[bjornbytes]

This is implemented on the dev branch.