change pixel-serializer to support reencoding existing data

Trying to evolve this interface so it can
- support rich set of backend-encoders (including ones like ETC1 that can cheaply convert to KXT
- allow for encoding images as well as bitmaps (e.g. for picture serialization)
- perhaps replace SkImageEncoder as an API (assuming we create a factory that returns a serializer given a format)

BUG=skia:

Review URL: https://codereview.chromium.org/1373683003
11 files changed