Enable fp16 visuals and fbconfigs
Continuation of: https://patchwork.freedesktop.org/series/54759/
This series enables fp16 fbconfigs and visuals by leveraging existing off-screen rendering support.
These formats can be used in conjunction with EXT_surface_SMPTE2086_metadata (not yet implemented by any drivers) to support EXT_gl_colorspace_scrgb / EXT_gl_colorspace_scrgb_linear, used in places like Android wide color gamut.
As an easy smoke test I have a modified version of kmscube: https://gitlab.freedesktop.org/strassek/kmscube/commits/fp16