Recognize typeless BC4/BC5 (RGTC) compressed texture formats
Add preliminary support for grayscale PNG loading

Grayscale PNG images (8-bit only) can now be loaded. Saving of
grayscale PNGs has not yet been tested.

A further improvement would be to support loading of 16-bit
grayscale images or indeed 16-bit RGB/RGA, which is mostly already
internally supported in texgenpack.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>