lock special-raster while the caller's bitmap is in-scope

previous CL that removed the specialimage subclass that wrapped the original image: https://codereview.chromium.org/2164073004

BUG=skia:5574
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2183483003

Review-Url: https://codereview.chromium.org/2183483003
1 file changed