All of the operators in vertex paint mode didn't work properly with
the new color attribute system. They only worked on byte color type
attributes on the face corner domain.

Since there are four possible combinations of domains and types now,
it mostly ended up being simpler to convert the code to C++ and use
the geometry component API for retrieving attributes, interpolating
between domains, etc. The code changes ended up being fairly large,
but the result should be simpler now.

This patch is targeted at 3.2, in combination with rB3545d8a50083