[PDF] Add support for SrcIn, SrcOut, DstIn, DstOut xfermodes.

[PDF] Add support for SrcIn, SrcOut, DstIn, DstOut xfermodes.

This change uses the soft mask (aka soft clip) functionality of PDF to implement the xfermodes.  It has to put existing content (dst) into a form xobject as well as putting the new (src) content into a different form xobject.  It then draws one of them with the other as the soft mask.
To accomplish this, we add a call to finishContentEntry after each call to setUpContentEntry - this is kind of a hack, but I don't see a better way to extract src.
Unfortunately, soft mask is specified in the Graphic State PDF object (and not in the form xobject), so when handling one of these modes, we add a one time GS object to set the soft mask and invoke a simple GS to reset the soft mask when done.

Review URL: http://codereview.appspot.com/4496041

git-svn-id: http://skia.googlecode.com/svn/trunk@1320 2bbb7eff-a529-9590-31e7-b0007b416f81