Update SkSpecialImage API

This adds some temporary compatibility features:
   internal_getBM & internal_fromBM - to convert back & forth from Bitmaps in the ImageFilters
   internal_getProxy - to allow the special images to interface with the current ImagerFilter system

It also adds a unique ID, opaque flag and size method.
   The unique ID and size method are needed for the image filter's caching system.
   The opaque flag is need for swapping back & forth from Bitmaps

These are all calved off of https://codereview.chromium.org/1695823002/ (Get OffsetImageFilter really working with SkSpecialImages) which actually makes use of them.

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Review URL: https://codereview.chromium.org/1705113002
6 files changed