implement isABitmap for imageshader, return localmatrix for bitmap's impl

For imageshader, I only return true if the image is explicitly raster-backed. I do not return true for texture, nor for generator (i.e. lazy/picture) backed. Is that ok?

BUG=skia:5592
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2197323002

Review-Url: https://codereview.chromium.org/2197323002
8 files changed