[graphite] Get ImageProviderTest fully working

This is mainly some small mipmapping fixes

Change-Id: I73707309acf50f31e550c9365d5bbc24805c7d71
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/581356
Commit-Queue: Robert Phillips <[email protected]>
Reviewed-by: James Godfrey-Kittle <[email protected]>
6 files changed