base

Implement support for PorterDuff's blending modes.

The blending modes are currently hooked up only when drawing filled rects but the
code is reusable for other primitives. This will allow implementation of saveLayer().
This method is required to support the fade effects used throughout the standard
Android UI.

Change-Id: I396023d123436f16cdafc606e358e4eb80c9df2c