Bug fix #34213 (2)

Adding non-RGBA float buffers as texture crashes MipMap code.
Images with less than 4 channels not supported (yet). Not sure
if we ever should even... normals, Z, vectors, UV, Index, all
don't like downsampling.



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54502 954f8c5b-7b00-dc11-b283-0030488c597c