Allow use/generation of MIP maps with YUVA images and strict subsets.

Strict subsetting is odd to mix with MIP maps, but we allow it for
all other images. At least be consistent. Long term we should remove
this feature and require users to make a subset image with it's own
MIP levels if they want to strictness + MIPs.

Bug: skia:10481
Change-Id: Ia12b60ef8d65a10bdf32f713daf77c366949ac26
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374598
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed