Add api for passing mipped hint into ImageGenerator onGenerateTexture

This does not actually add any additional functionality to the generators.
Once this lands I will enable the generators one at a time to more easily
monitor the effects of each one.

Bug: skia:
Change-Id: I382a1acfaebcbf9ad44c9873b87cdbbe02a13602
Reviewed-on: https://skia-review.googlesource.com/57083
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
12 files changed