ccpr: Polyfill buffer mapping when not supported

Change-Id: I62880a83d9b59d42c6491125e2a62338d2ce757f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/279200
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
8 files changed