case where the source format is PIX_FMT_GRAY8.

This is required as PIX_FMT_GRAY8 has been declared as a paletted
format in FFmpeg r22191, fix GRAY8 -> RGB conversion.

git-svn-id: file:///var/local/repositories/mplayer/trunk/libswscale@30826 b3059339-0415-0410-9bf9-f77b7e298cf2