Skia's dithered gradient shaders cost ~10ms per full-screen pass on
Mali-class TV GPUs: the two spotlight scrims alone were ~20ms of a 27ms
raster frame, while flat blended quads at the same coverage are ~free.
Bake axis-aligned LinearGradients once into cached 1x1024 premultiplied
strip textures drawn as stretched quads (shader fallback for unsupported
shapes and the first frame). Converted the spotlight, TV detail backdrop,
app-bar scrims, rail bleed, person-card overlay, and the video-controls
scrim, which now also keeps one widget type across hasFrame flips so the
controls subtree survives in-place source switches.
Scripted-browse on a Mali-G31 box: draw p50 27.5ms -> 7.1ms, swap block
20.3ms -> 0.9ms, janky draws 69% -> 2%; screenshots pixel-identical.
