perf: A bunch of render path improvements

[Crecket]

Render path improvements

• Coalesce redraws into one per frame. draw() is now a requestAnimationFrame scheduler and the old synchronous body is drawNow(), so all ~46 call sites coalesce without being edited. resize() stays synchronous to avoid a blank frame on every ResizeObserver tick.
• Cache CSS custom properties. renderer.js, grid.js and overlays.js read getComputedStyle(document.documentElement) on every paint; now cssVar() memoises them, invalidated from applyTheme().
• Decode tiles off the main thread. createImageBitmap() for same-origin tiles, new Image() plus img.decode() for cross-origin ones — fetch is subject to CORS where an <img> load is not. TILE_CACHE becomes a 256-entry LRU that close()s evicted bitmaps instead of growing unbounded.
• Cap devicePixelRatio at 2 via a shared renderScale(), also used by the contour raster so it cannot disagree with the canvas.

Scenario	Painter ms, main	Painter ms, branch	Per frame, main	Per frame, branch
Drag pan, 600 moves	154.1	62.1	1.28 ms	0.52 ms
Wheel zoom, 300 events	126.8	47.5	2.11 ms	0.79 ms
Cold tile pan, zoom 6	141.4	51.8	2.36 ms	0.86 ms
Drag pan at DPR 3	176.1	68.1	1.47 ms	0.57 ms

[apollyon-sys]

Thanks for the PR - I went through it and the changes look good to me. The render-path optimizations are nicely scoped and I don’t see anything blocking the merge. I’m going to merge this one first since #16 also touches renderer.js / overlays.js

[Crecket]

In my personal version I draw "deadzones" where low arc shots can't reach and height contour lines and more and in that version I've added even more improvements + offloaded some things to webworkers to speed it up even more, so it's at the point where I can maintain 120fps on my macbook. I'll see if I can port some of those over aswell once more of my PRs are merged

[Crecket]

If you want to compare: Open https://wardogs-map.olm.pet, toggle on all layers and then in the devtools run localStorage.setItem('wardogs-fps-meter', '1') and then refresh the page. It should show a FPS counter in the bottom right