Chrome text rendered blurry on Retina: all chrome/canvas text goes
through the offscreen 2D-canvas raster path (drawBrowserText), which
rasterized glyphs at CSS (1x) resolution. Those bitmaps are then drawn
onto the CanvasKit surface while a scale(dpr, dpr) transform is active,
magnifying them dpr-x — half-resolution on 2x displays.
Supersample the offscreen text canvas by the device pixel ratio and
composite it back at 1/dpr so glyphs land at native device resolution.
Thread the real dpr from Rust (CanvasKitBackend::new + resize_for_display)
into the bridge via a new setDpr method; cache key includes dpr and
stale bitmaps are dropped on dpr change.
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