Dawn: implement mipmap generation.

Dawn does not support automatic mipmap generation, so use mip-to-mip
downsampling.

Change-Id: I71e1808d78f45eee68df7f124100f5b563f29da3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319736
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Stephen White <senorblanco@google.com>
7 files changed