libcolord: Add cd_icc_create_default() to use the default sRGB profile

This is sometimes a lot faster than loading the on-disk sRGB profile due to
things like transalations and other metadata that is not needed for the transform.
trivial: Only encode two decimal places when writing the coverage in cd-fix-profile
libcolord: Export cd_icc_utils_get_coverage() for comparing two profiles
trivial: Use cd_icc_utils_get_coverage() in cd-fix-profile
Set 'GAMUT_coverage(srgb)' when generating standard space profiles

Although, obviously not for sRGB itself.
libcolord: Always calculate the fallback checksum when using CdIccStore

Using profiles without an embedded profile-id is still very common, and given
the CdIccStore relies so much on the checksum for duplicate checking we should
enforce that at *least* this flag is set by default.