BF: Some (but not all) greyscale images were being misinterpreted by PIL

Some were being detected as RGB and some left as L which was then
causing errors in type conversions.

Force all to be greyscale if alpha mask and force all to RGBA if image/tex
BF: further fixes to greyscale image stims, including numpy arrays

These should be processed as gl.floats and then colored, which wasn't
working correctly