Scalable spelling/grammar markers
Replace the inlined bitmap markers with vector drawings, to improve rendering
at arbitrary zoom levels. Use cached SKP recordings for marker fragments, and
a picture shader for tiling in X.
In order to minimize disruption, this change aims to keep rendering as close
to the original bitmaps as possible. This can be revisited in the future
(to e.g. improve dot spacing/distribution on Mac).
Performance should be comparable to the current bitmap shader impl: Skia
caches picture shader tiles, so for a given zoom level all markers should be
hitting the same pre-rasterized cache entry (and essentially devolve into