This adds helpers to ISL to convert an isl_color_value to and from
binary data encoded with a given isl_format.  The conversion is done
using ISL's built-in format introspection so it's fairly slow as format
conversions go but it should be fine for a single pixel value.  In
particular, we can use this to convert clear colors.

As a side-effect, we now rely on the sRGB helpers in libmesautil so we
need to tweak the build system a bit.  All prior uses of src/util in ISL
were header-only.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>