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

See https://buganizer.corp.google.com/issues/129117085

Change-Id: I7c532e63a80b118fb2416c572b8e0d2abf8cf59a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209166
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211600
Commit-Queue: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
28 files changed