Fix GrBackendTextureImageGenerator to hold context lock till all proxies are gone

This fixes a bug where if we created two proxies for the same context. We would
release the context lock after one of the proxies was released instead of
waiting for all proxies to be released.

Bug: skia:
Change-Id: Ia6ed8148abb029bd1f95c85bc3d3ef003e8de408
Reviewed-on: https://skia-review.googlesource.com/102322
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed