Make a proof-of-concept patch for split Color Smudge and Color Rate options

This patch addresses bugs 329557 and 420600. Bascially it ensures
that the final opacity is greater or equal the one set in opacity
option. Though since we have a feedback loop in colorsmudge brush,
this is not realy precise.

Limitations of the patch:

1. It only works in "Smearing" mode. Dulling mode is just not implemented
2. It is slower than the original implementation (not fixable)
3. It works weirdly when the source layer has transparency. Please test on layers filled with opaque color.