Web performance: light animation, gestures on a compositor layer, no forced layouts

Measured with the new tools/bench_web.py on the web Studio's house demo, CPU slowed 6x:

- Idle: running CSS animations repainted the whole drawing every frame (99% of the main
  thread in a window). Animations now step on about ten times a second by default
  ("light", 5% idle); the new toolbar button switches light / smooth / off and the browser
  remembers it; [build] animation sets the default for built pages; view.animation().
  Pan and zoom gestures pause them. Status LEDs pause off-screen and with the rest now.
- Pan 28 -> 10 ms, zoom 69 -> 11 ms per step: gestures move the drawing as one compositor
  layer (transform) and set the viewBox once they end; --iso-k is set on the tags group
  only and only when it changes; the stage box is read once per frame.
- Hover 22 -> 15 ms per move: the tooltip is placed once per frame on its own layer and
  measured only when its text changes; the Studio's x/y readout updates ten times a second
  in a contained box; hover work in the Studio runs once per frame.
- Long object lists skip layout of rows out of sight (content-visibility).
- WASM built with opt-level 3: engine render 11.7 -> 9.4 ms for 10% more download.

Checks: Studio 102/102 (animation modes, pan as a layer, contained readout), web 87/87.

Cargo.toml

@@ -38,10 +38,11 @@ wasm-bindgen = "=0.2.129"
 lto = "thin"
 codegen-units = 4
 
-# the web Studio's engine (tools/build_web.py): small over fast
+# the web Studio's engine (tools/build_web.py): fast over small (opt-level 3 renders
+# about a fifth quicker than "s" for a tenth more download, 0.9 MB gzipped)
 [profile.web]
 inherits = "release"
-opt-level = "s"
+opt-level = 3
 lto = true
 codegen-units = 1
 panic = "abort"