Make subset param to texture generation API non-optional

We were always already supplying this, makes it simpler

BUG=skia:

Change-Id: I36ac35205df5ab2a0fb7ec26e83ddb1547154816
Reviewed-on: https://skia-review.googlesource.com/5778
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed