Remove subset parameter from making encoded/generator images

Since subsetting may require rasterizing/resolving the generator, we may
also need access to the GrDirectContext. To simplify apis, rely on
makeSubset() for that.

Related: https://skia-review.googlesource.com/c/skia/+/305970

Change-Id: I1980e3c823fb6cf54f197c350942c2f82b03e20f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306136
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
11 files changed