One of the regressions from OOP-R is that we've moved work from the renderer raster worker threads and onto the gpu main thread.  In cases where the gpu main thread was already overloaded, this can regress mean frame times.

We need some solution in the gpu process (likely SkDDL and multiple threads) to regain some of this paralellization.

See also: https://bugs.chromium.org/p/chromium/issues/detail?id=860264#c7
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