Fix for texture preview render with show alpha enabled, now it shows no
transparency when use alpha and calculate alpha are disabled.

It's a bit arbitrary as it's not possible to fully visualize the output
of a texture, that depends on how it's used in the texture stack, but this
is a bit more in line with what you might expect.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55026 954f8c5b-7b00-dc11-b283-0030488c597c