Switch some ints to unsigned (they can only have positive values, this allows

Switch some ints to unsigned (they can only have positive values, this allows
compiler to optimize some math from mul/div to shr/shl). Also add a cast to
uint32_t when calling decodeplane32(), this silences a compiler warning.
Lastly, in decodeplane8/32(), flatten a double-loop into a single-loop and
calculate the length once before entering the loop instead of during every
iteration (since it doesn't change).

Patch by Sebastian Vater <cdgs.basty googlemail com>.

Originally committed as revision 22974 to svn://svn.ffmpeg.org/ffmpeg/trunk