Improve texture painting alpha channel

Right now the only options to specifically modify the alpha channel is to use the Add Alpha, or Erase Alpha blend modes. There seems to be no way to paint an absolute alpha value, it's always relative.

There are two solutions to this:

1. Add alpha to the colour we're painting with, then rely on the Affect Alpha setting to determine if we are painting to the alpha channel. This is the best solution as we can then have access to all the other blend modes.
2. Add a Mix Alpha blend mode, that lets us draw a specific alpha value to the texture, less complete solution, but may be easier to implement though.