FB-specific extensions should be optional, not required

[BattleAxeVR]

Currently XR_FB_SWAPCHAIN_UPDATE_STATE_VULKAN_EXTENSION_NAME is in XrAppImplVulkan::getXrRequiredExtensions(), and thus Vulkan doesn't run on other non-Quest VR headsets.

It works fine on GL ES mode on other headsets though, which don't have this.

For ex: XR_FB_PASSTHROUGH_EXTENSION_NAME is checked in the enumerated list of all supported extensions, but not required:

This is the ideal pattern to use for optional or vendor specific features, I think. It's also useful for things like eye- or face-trackingwhich may or may not be enabled by the user thus unavailable, even on Quest Pro.

[rokuz]

Extensions were refactored. XR_FB_SWAPCHAIN_UPDATE_STATE_VULKAN_EXTENSION_NAME is optional now.