(Mostly) Retract GrRenderTarget from SkGpuDevice

This gets us most of the way to having SkGpuDevice exclusively use a GrDrawContext instead of a GrRenderTarget. There are a few other refactorings (e.g., rm need for fLegacyBitmap and accessRenderTarget) before the GrRenderTarget can be completely removed.

Has calved off:
https://codereview.chromium.org/1925313002/ (Tighten up SkSpecialSurface factory functions)
https://codereview.chromium.org/1925803004/ (Add sk_sp to SkSurface_Gpu and SkGpuDevice)
https://codereview.chromium.org/1956473002/ (Retract GrRenderTarget a bit within SkGpuDevice)
https://codereview.chromium.org/1979913002/ (Rename GrDrawingMgr::abandon to wasAbandoned & add a matching entry point to GrDrawingContext)
https://codereview.chromium.org/1982583002/ (Add isUnifiedMultisampled entry point to GrDrawContext)

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Review-Url: https://codereview.chromium.org/1930013002
7 files changed