Force BackendTextureImageGenerator to work on only 1 consumer context

Bug: skia:
Change-Id: I83d3e4af6f20c877c541964fdd489434f6a62b25
Reviewed-on: https://skia-review.googlesource.com/93200
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed