ImagePaint Refactoring:

ImagePaint Refactoring:
- ImagePaint now uses ImBuf directly, and the rect blending functions
  were moved into the imbuf module.
- The brush spacing, timing and sampling was abstracted into brush.c, for
  later reuse in other paint modes.

Float ImagePaint support.

Textured Brushes:
- Only the first texture channel is used now.
- Options for size and offset should be added, but need to find some space
  in the panel, or add a second one ..