Remove more calls to SkCanvas::flush() and SkSurface::flush() (flutter/engine#43902)
Skia has removed flushing from both
[SkCanvas](https://skia-review.googlesource.com/c/skia/+/716476) and
[SkSurface](https://skia-review.googlesource.com/c/skia/+/698237). This
migrates the calls to use GrDirectContext directly (or removing no-op
flushes for Raster canvases/surfaces).
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