color: add supported_feature event

This makes the support for a few requests to be explicitly advertised and unavailable otherwise. This allows compositors to support only enumerated transfer functions and primaries, instead of requiring them to support the arbitrary flavors. This may be useful for hardware-specific compositors that cannot use programmable shading.

set_mastering_display_primaries is a bit more special. The support for it is split into two features: basic support, and extended color volume support. Extended color volume support requires handling color values outside of the range [0.0, 1.0] which requires special care from compositors. Silently clipping out-of-range values may or may not be acceptable, so clients should be aware whether it works. If clipping is acceptable, they can try an image description without extended color volume.

This conflicts with !36 (closed) slightly, but after typing this up, I like this better.