Auto merge of #16731 - glennw:update-wr-borders-nearly-done, r=pcwalton
Auto merge of #16731 - glennw:update-wr-borders-nearly-done, r=pcwalton

Update WR (remove old border path, composite fixes).

* Support border corner transitions with new path.
* Remove old border rendering code and shaders.
* Bug fix for mix-blend-mode / composite tasks.

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