For PAL8 files, load them in an RGB24 frame instead.

This allows loading tiff files with libav without waiting for libav 11 but
it will create colour files by default, even though the image would be
grayscale only.

libav 11 supports grayscale tiff files as grayscale rather than palettized,
but until then, this workaround should work.
Add a note about dependencies suggesting to use libav 11 if available.