Fix drawing xfermodes to PDF.

Cherry pick of in Skia
[PDF] Improve complex xfer mode support.

Xfer mode applies only to the shape of the source drawing, not everything in the
clip as in currently implemented.  It's just that the current gm examples draw a
shape that fills the visible layer.


Change-Id: I9f82aeb09ead0d7e600590264a594262ec6c6282
4 files changed