avformat/mov: Do not compute a grayscale palette for cinepak in mov.

This was never done for avi files, the decoder always produced
output no matter if a palette was computed or not.
Since a non-standard palette was needed, this simplifies the code.
See issue 1067 and ticket #165.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Allow stream-copying grayscale mov files.

This reverts 0de2157f / r12272.
Fixes ticket #3215.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>