Add secret draw-behind method

This is a variant of drawPaint but is automatically clipped
to the bounds of the most recent saveBehind buffer (axis-aligned bounds).

No public exposure.

Impl is pretty simple (its a variant of drawPaint)
- find the most recent saveBehind device bounds
- if there is none, draw nothing, else
  - temporarily intersect the device's clip with that bounds
  - drawPaint
  - restore the clip


Change-Id: I7c532e63a80b118fb2416c572b8e0d2abf8cf59a
Commit-Queue: Mike Reed <>
Reviewed-by: Brian Salomon <>
Commit-Queue: Derek Sollenberger <>
Reviewed-by: Derek Sollenberger <>
28 files changed