Add support for SkImageGenerator creating external textures

Copy the SkImageGenerator texture if tiling is needed and
original texture target is GR_GL_TEXTURE_EXTERNAL.

Bug: skia:
Change-Id: I98f5acc3883e2060b1a35f80633b02b08a706107
Reviewed-on: https://skia-review.googlesource.com/18268
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stan Iliev <stani@google.com>
14 files changed