Make SkImageGenerator::getPixels() return an enum.

The new enum describes the nature of the failure. This is in
preparation for writing a replacement for SkImageDecoder, which will
use this interface.

Update the comments for getPixels() to specify what it means to pass
an SkImageInfo with a different size.

Make SkImageGenerator Noncopyable.

Leave onGetYUV8Planes alone, since we have separate discussions
regarding modifying that API.

Make callers of SkImageDecoder consistently handle kPartialSuccess.
Previously, some callers considered it a failure, and others considered
it a success.

BUG=skia:3257

Review URL: https://codereview.chromium.org/919693002
8 files changed