Pin color type when a cross-context image is constructed

Fixes issues with gray images that may be incorrectly re-interpreted as
alpha when re-wrapped. (https://github.com/flutter/flutter/issues/15600)

Change-Id: I4a78466073e14d212108d168eed0b2df1bc92ffe
Reviewed-on: https://skia-review.googlesource.com/116484
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed