Fix off by one typo in MMX optimized clipmask blending.
Fix off by one typo in MMX optimized clipmask blending.

darcs-hash:20100514221709-f546f-08a8e9031d88e1531bc77a8360b86b39a936560e.gz

nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf