Alpha-blitting revisited: full alpha-pane based blitting

Alpha-blitting is now not just a special blitting case, it is now a basic feature that comes with color values that hold alpha information (Color8A, ColorRGB32) as well as with the Blitbuffers with according layout. "adding" values is now integer-based, which speeds it up by a factor of 2.5 (on x86_64 at least).

Font handling is switched over to use the map as given by freetype, without conversion to 4bpp. So our text rendering can do color in the future (all graphics primitives should now be able to do this) and got rid of background rendering.