compositor: Check the protected content bit, and return a non-success code if it's set. Supporting this is optional in OpenXR, but lack of support must be reported to the application.
OpenXR: Add support for creating swapchains with depth formats and submitting depth layers. The depth layers are passed through to the compositor, but are not used yet.
Rename functions and types that assumed the native graphics buffer handle type was an FD: in `auxiliary/vk/vk_helpers.{h,c}``vk_create_image_from_fd` -> `vk_create_image_from_native`, in the XRT headers `struct xrt_compositor_fd` -> `xrt_compositor_native` (and method name changes from `xrt_comp_fd_...` -> `xrt_comp_native_...`), `struct xrt_swapchain_fd` -> `struct xrt_swapchain_native`, `struct xrt_image_fd` -> `struct xrt_image_native`, and corresponding parameter/member/variable name changes (e.g. `struct xrt_swapchain_fd *xscfd` becomes `struct xrt_swapchain_native *xscn`).
