Improve comments
Remove TIFF CMYK support

32bpp data was being incorrectly received as CMYK data if the CMYK flag
was set.  So for now, do not write CMYK-encoded TIFF files.
Fix auto color depth detector to check for complex alpha

This way, if an image has complex alpha but <= 256 colors, it will still
be written as 32bpp
Fix GDI+ interface to write 32bpp files correctly