Baking assumed that color attributes could only have two configurations:
float color data type on vertices, or byte color type on face corners.
In reality the options can be combined to make four total options.
This commit handles the four cases explicitly with a somewhat
more scaleable approach (though this should really be C++ code).

This commit also changes some related error messages, tooltips,
and an enum name, in order to make the functionality more obvious.

A version of this change without the UI text changes can be committed to 3.2.