[Feature] Implement fixed-foveated rendering

[kanerogers]

Background

Speaking with an Oculus Engineer, one of the biggest ways we can get an improvement out of our rendering pipeline is to use Fixed-Foveated rendering.

Facebook has an OpenXR extension that should make this very easy to implement.

[kanerogers]

Background

OpenXR

Implementing FFR seems to need a couple of dependent extensions in OpenXR

• XR_FB_swapchain_update_state
• XR_FB_foveation
• XR_FB_foveation_configuration
• XR_FB_foveation_vulkan

The setup seems to look something like this:

1. Modify our call to xrCreateSwapchain to include an XrSwapchainCreateInfoFoveationFB structure
2. Create a xrFoveationProfileFB with xrCreateFoveationProfileFB with a next pointer to a XrFoveationLevelProfileCreateInfoFB
3. Immediately call xrUpdateSwapchainFB with an XrSwapchainStateFoveationFB structure that points to the XrFoveationProfileFB
4. When we call xrEnumerateSwapchainImages, if we include XrSwapchainImageFoveationVulkanFB, we should get a VkImage we can pass down to our renderpass.

Vulkan

And then on the Vulkan side:

• VK_EXT_fragment_density_map2

TODO

• Enable the relevant extensions in OpenXR
• Change xrCreateSwapchain call
• Call xrCreateFoveationProfileFB
• Call updateSwapchainFB
• Get foveation image when enumerating swap chain images
• Hide FFR behind a device flag so that we can run on the simulator

[kanerogers]

This was closed in #415