Discard atlas after every MultiPictureDraw::draw

This is intended to prevent ghosting on tiled architectures.

This CL also defers creation of the atlas (and its texture) until it is actually needed.

Committed: https://skia.googlesource.com/skia/+/6d5b5455743414ddb11d2b8c1fe9d7959f2b853d

Review URL: https://codereview.chromium.org/678403002
6 files changed