Make openxr backend work for non-HL2 devices

[Manishearth]

It's been written specifically for HL2. We should catalog all the cases where that's been done, and expand support where necessary so that we can test with other Windows MR devices.

[Manishearth]

Some issues:

• We only support one image format

• While we currently don't support the actual floor level, if we start doing so we will have to deal with the case where the device doesn't support floors.

• We assume that the device's controller is simple_controller compatible

• We hardcode the additive blend mode in openxr

[msub2]

Taking stock of these now after having started to experiment with this on my Win10 machine:

• When requesting a session using SteamVR's OpenXR runtime, DXGI_FORMAT_B8G8R8A8_UNORM is not in the list of available formats. DXGI_FORMAT_B8G8R8A8_UNORM_SRGB is though, and appears to work fine (though looking at Avoid needing to gamma correct for OpenXR #136 I suppose it's incurring some gamma correction, not sure what the performance impact is there). The full list of available formats it gives is:
  • DXGI_FORMAT_R8G8B8A8_UNORM_SRGB
  • DXGI_FORMAT_B8G8R8A8_UNORM_SRGB
  • DXGI_FORMAT_R32G32B32A32_FLOAT
  • DXGI_FORMAT_R16G16B16A16_FLOAT
  • DXGI_FORMAT_R10G10B10A2_UNORM
  • DXGI_FORMAT_D32_FLOAT
  • DXGI_FORMAT_D16_UNORM
  • DXGI_FORMAT_D24_UNORM_S8_UINT
  • DXGI_FORMAT_D32_FLOAT_S8X24_UINT
• Haven't poked at this yet but willing to take a look
• Are there any devices on the market that aren't compatible with the simple_controller? It does seem to be trying to be the minimum viable control scheme after all.
• Seems like this was done in Use supported environment blend mode in openxr backend. #132