Fix #35278: textures nodes Mix RGB node did not have a Use Alpha option like
the compositing node. Note the shader nodes can't have this because color
sockets there are only RGB, not RGBA.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56658 954f8c5b-7b00-dc11-b283-0030488c597c