Use a faster way to compute 255-val: Instead of creating a vector of

Use a faster way to compute 255-val: Instead of creating a vector of
all 255s, and then doing the subtraction, nor of the vector with itself: saves
one instruction and a register.
Patch by Graham Booker % gbooker A tamu P edu%

Originally committed as revision 9340 to svn://svn.ffmpeg.org/ffmpeg/trunk