Dynamic Resolution

[bjornbytes]

Implements dynamic resolution in lovr.headset, so the render resolution will scale automatically based on performance to help keep the frame rate high/consistent.

This uses two different OpenXR extensions:

• XR_EXT_view_configuration_views_change is an event that the runtime sends if the user's preferred rendering resolution changes, like the "render scale" slider in SteamVR settings. LÖVR listens to this event now and resizes the headset texture as the slider moves. (This was a long-standing feature gap between OpenXR and OpenVR).
• XR_META_recommended_layer_resolution is "real" dynamic resolution. LÖVR asks the runtime for a recommended resolution every frame, and it scales dynamically with performance.

Other notes:

• You can control whether dynamic resolution is active with t.headset.dynamicresolution, which defaults to true.
• This probably replaces/deprecates t.headset.supersample, although currently I have it so the supersample factor is still applied if neither of the extensions are supported. Maybe it's still useful for the simulator? It would be nice to reduce the amount of moving parts though.
• lovr.headset.getDisplayDimensions returns the dynamic resolution.
• The dynamic resolution updates when lovr.headset.update is called.
• The Pass you get from lovr.headset.getPass has its viewport/scissor set to the dynamic resolution.
• I have to do some lovely hacks in lovr.mirror to render the sub-rectangle of the headset texture. Pass:setQuad can't come soon enough.
• In general it's a huge pain to use the headset texture for anything afterwards, since you have to lug around the viewport size any time you want to sample the texture (as shown in lovr.mirror). In situations where this is too annoying, you could disable dynamic resolution.

Is this worth it? I guess so. It's better than setting t.headset.supersample to something arbitrary and hoping for the best.

Closes #956

[bjornbytes]

typo (uppercase R)

[bjornbytes]

I don't love the dynamicresolution name. It's too long. t.headset.supersample = 'auto'? t.headset.autosize? t.headset.renderscale = 'auto' | number?

[bjornbytes]

Ugh, Meta returns XR_ERROR_SESSION_NOT_READY if you query the recommended resolution before the session transitions to READY... In that case we disable dynamic resolution. We'd have to delay swapchain creation to after receiving the READY event, which is gross. SteamVR doesn't do this and will happily return a recommended resolution at startup.