Replays rebuild every frame, and since #304 they make the map's colour
mappers inside that per-frame build. A mapper had no equality, so each
frame's looked new to flutter_svg, which reloaded and redrew both maps
(the map and its revealed copy) every frame and lost the engine's cached
raster of them. Replay playback went from 4.8 to 8.2 ms of raster a
frame (144 Hz budget: 6.9 ms); with equality it is back to 5.0 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>