[PDF] Add support for xfermodes / blend modes.

[PDF] Add support for xfermodes / blend modes.

- Change SkGraphicState to track and set the blend mode (xfermode) for modes built in to PDF (non porter duff modes + src over).
- Add SkXfermode::asMode() to retrieve xfermode as an enum for non porter duff modes.
- Move SkXfermode.cpp around a bit to support asMode() -- Generally move utility functions toward the top of the file.
- Make SkPDFFormXObject an isolated transparency group, as it used for saveLayer, which draws on transparent, not the device background.
- Set the graphic state in drawDevice and drawBitmap in order to get the right xfermode and alpha.

Review URL: http://codereview.appspot.com/4131043

git-svn-id: http://skia.googlecode.com/svn/trunk@774 2bbb7eff-a529-9590-31e7-b0007b416f81