More painting fixes:

* 2D image painting with textures that contained alpha did not work correctly,
  had been broken for a while.

* 2D image panels texture (mask) panels showed wrong buttons for texture overlay.

* Texture map mode 3D now also uses masking, like Tiled and Stencil the texture
  does not move along with the brush so it works fine.

* 2D image paint View mapping did not work correct, especially noticeable with
  Rake rotation.

* Masking is now disabled for the smear tool, this can't really work because
  the original image is constantly changing and gave artifacts.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56417 954f8c5b-7b00-dc11-b283-0030488c597c