[PDF] Add clip support and some optimizations for "complex" xfer modes.

[PDF] Add clip support and some optimizations for "complex" xfer modes.

For Clear, Src, Dst/Src-In/Out, we have to consider the current clip, and
potentially draw Dst, clipped to the inverse of the current clip before
doing the operation of interest.

For clear or src, if we haven't drawn anything, or the clip is empty, there's
nothing to be done.
For Src/Dst-In/Out, if either is empty, the result is empty.

Review URL: http://codereview.appspot.com/4538082

git-svn-id: http://skia.googlecode.com/svn/trunk@1407 2bbb7eff-a529-9590-31e7-b0007b416f81