Different approach to fixing gpu blurs on platforms that "useDrawInsteadOfClear"

This CL reverts https://skia-review.googlesource.com/c/5148/ (Fix gpu blurring on platforms that "useDrawInsteadOfClear") (all the worstCaseWidth/Height stuff) and adds a new GrRenderTargetContext entry point (absClear) to specify clears that can't be discarded or altered.

BUG=skia:

Change-Id: I18b1373ecf4a153ca8c0f290ab8b1d00770426da
Reviewed-on: https://skia-review.googlesource.com/5484
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
15 files changed