Mark wrapped textures imported into SkImages as "read only".

Read only textures and proxies fail writePixels, as copy dsts, and mip
regeneration.

Bug: skia:8509

Change-Id: Iaa0b473cc9a9930fde3ef0e91373d5040650de35
Reviewed-on: https://skia-review.googlesource.com/c/174316
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
53 files changed