Change tgq_decode_mb function arguments from using int8_t to uint8_t, this does
not really change the code (x86_64/gcc-4.3.3 results stay unchanged) but avoids
several casts and an aliasing violation that broke decoding at least on
PPC64/gcc-4.1.2.

Originally committed as revision 18541 to svn://svn.ffmpeg.org/ffmpeg/trunk
