[graphite] Move TextureProxy ctor impl into to resolve forward decl

Bug: skia:12466
Change-Id: I273fbb51730d85cda6921747e72ace2b9d7601e1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/462880
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed