Add GPU implementation of rescaling for SkSurface::asyncRescaleAndRead.

The only new capability added is bicubic downscaling but this opens up
more possibilities to add specializations to reduce passes in the GPU
implementation.

Bug: skia:8962
Change-Id: I4ff2e89b24df9dc15988d31ed10a3316b018e583
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215445
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
7 files changed