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.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1705113002
Review URL: https://codereview.chromium.org/1705113002
