remove (possibly slow) call to refEncoded in getDeferredTextureImageData

remove (possibly slow) call to refEncoded in getDeferredTextureImageData

- explicitly reject already-texture-backed and picture-backed

Needed to add a virtual to image_base and generator to distinguish
generators that can (or cannot) natively "generate" on the gpu (e.g. pictures)
Bug: 646089
Change-Id: I3aea22f89b31009ecbb3bd50d88512e6532f0a0f

Change-Id: I3aea22f89b31009ecbb3bd50d88512e6532f0a0f
Reviewed-on: https://skia-review.googlesource.com/15765
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>