Blender Git Loki

* brush and airbrush match The Gimp, where the total color applied to a pixel during a stroke is limited by the brush opacity. With the airbrush option enabled it will accumulation over the opacity level (as it did before, irrespective of the settings).
* float buffer support for all painting operation and undo.
* only run brush_sample_tex() for textures brushes.
* redraw the brush outline even when nothing is painted.