Retract GrRenderTarget a bit within SkGpuDevice

Split off of: https://codereview.chromium.org/1930013002/ ((Mostly) Retract GrRenderTarget from SkGpuDevice)

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1956473002

Committed: https://skia.googlesource.com/skia/+/d215a95882485026d176a81bf58871276b21c89c

Review-Url: https://codereview.chromium.org/1956473002
3 files changed