Besides better matching Viz's behavior this also reduces a lot of choppiness in the composition RenderTask DAG.
In the previous approach DDL draws and compositing draws would be interleaved resulting in a lot of render target swaps.
This necessitated some reorganization bc I wanted to reuse PromiseImageCallbackContext to manage the tiles' promiseImages.
Change-Id: I513bf060a69ff2bfe0e7b82ae72f149dfede632e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285056
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
11c6767a17