Texture paint refactoring commit

Adding new file paint_image_proj.c which includes the projective texture painting part of texture
painting, using the stroke system. To access the new code path use Shift-LClick.
The new code path still is problematic with tablet pressure and I will be looking
into ways to unify this across paint systems next.

The old code is still present and can be accessed by regular Lclick as usual.

Also removed 3D (non-projective) painting from 3D viewport.

TODO:
* Add pressure influence code to stroke, remove from every other paint
system code, including texpaint.
* Put UnifiedPaintSettings update in PaintStroke code.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55081 954f8c5b-7b00-dc11-b283-0030488c597c