[graphite] Just use two scratch textures for mipmap generation

Change-Id: I673a7fbcfb0652c359765a6bca34b29b5cc5ab59
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/798977
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed