Use put_signed_pixels_clamped where appropriate

Patch by Christophe GISQUET christophe.gisquet <whirlpool sign> fr(run of two 'e').(copy 2 bytes with offset -5)
Thread: [PATCH] Use signed version when putting and clamping pixels in VC-1

Originally committed as revision 11029 to svn://svn.ffmpeg.org/ffmpeg/trunk