Tilemaps: a painted cell builds only the meshes it changes (#424)

A new paragraph on the fifth round: a patched list of tiles says what
it came from, and groupByTile and layerRows keep every group no changed
entry falls into, so its mesh is not built again. With the numbers of
the brush stroke on 60x60 and 120x120.

The closing sentence of the #348 paragraph about the mesh still
growing with the map is folded into the new one.

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

Tilemaps.md

@@ -392,9 +392,23 @@ A fourth round (#348) made a painted cell cost what is around it, not the whole
 
 Measured with the same map: one painted cell costs the scene's item list 3.5 ms on 60x60 and 4.0 ms
 on 120x120, against 3.4 ms without painting (headless, before 16.8 and 54.6 ms). A brush stroke on
-the map doubled to 120x120 runs at 42 fps instead of 28, p95 of a frame 52 instead of 92 ms. What
-still grows with the map is the mesh: a painted cell builds the geometry of every tile on that layer
-again, since the layer's tiles come as a new list and are grouped by tile afresh.
+the map doubled to 120x120 runs at 42 fps instead of 28, p95 of a frame 52 instead of 92 ms.
+
+A fifth round (#424) took out the meshes: a painted cell built the geometry of every tile on that
+layer again, since the layer's tiles came as a new list and were grouped by tile afresh.
+
+- A patched list of tiles says what it came from and which entries went and came
+  (`tilesPatch`). A cell around the painted ones that is drawn the same keeps its entry, and a
+  patch that changes nothing drawn hands back the very same list.
+- `groupByTile` and `layerRows` patch the groups of the list before (`patchedGroups`). A group no
+  changed entry falls into is the very same array, so its mesh is not built again. On a layer
+  sorted by Y that goes for each row and for the groups in it.
+- A tile new to the layer still works the whole layer out again, as before.
+
+Measured with the same stroke: meshes built per frame 21.9 to 0.94 on 60x60 and 40.7 to 1.22 on
+120x120, 49 to 51 fps and 42 to 46 fps. Painting now costs both sizes the same, about 10 fps below
+the same view without painting. What is left between the two sizes is drawing the larger map, which
+the view without painting shows as well.
 
 A frame still costs what it touches: an object standing **inside** many outlined cells is tested
 against each of them, so an outline of a hundred corners around every cell of a lake is dearer than