Dabs now go only into preview tiles of the shown level at every zoom,
including a level 0 preview overlay that the compositor shows in
place of the layer tile. The layer tiles are painted by the engine,
and releasing a stroke commits history without waiting for them.
Stroke opacity coverage is shared per tile across a stroke's flushes,
each engine job only replays the dabs that touch its tile, and undo
drops an open preview instead of painting it. Readers that need final
pixels settle explicitly; the renderer no longer does.
tp::Map::remove rehashed the whole table on every call, which made
applying thousands of finished tiles quadratic; it now rehashes only
when the table is mostly empty or full of deleted slots.
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