Mark GRAY8 format as pseudo-paletted.

This fixes that the GIF encoder crashes with it because
it has no palette.
And the arguments for the pseudopalette apply to gray8 as
much as to RGB8 etc.
In addition the changes required in lavfi should be needed anyway
when adding support for RGB8 etc.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
