Revisit HDR output metadata / enabling

In particular, VK_EXT_hdr_metadata seems to also imply HDR/PQ output, at least for AMD drivers. Presumably the same is true for nvidia - not that I can see their source code to check.

We should delete both prefer_hdr and pl_swapchain_hdr_metadata and merge them into a single abstraction for dynamically updating the preferred swapchain color space / hdr mode.