Pr fxlayer viewport culling (#3123)

## Description:

Reduce FX layer rendering cost by:
- Updating the offscreen FX buffer only when needed (and clearing it
once when FX ends).
- Drawing only the visible portion of the FX buffer to the main canvas
(viewport culling).
- Reusing `TransformHandler.screenBoundingRect()` as the single source
of truth for viewport bounds.
- 
## Changes
- `FxLayer`:
  - Track buffered frames and skip work when there are no active FX.
  - Use `performance.now()` for refresh timing.
- Draw only the visible map rect (clamp + small pad) instead of blitting
the full map-sized FX canvas.
- Compute the visible rect via `TransformHandler.screenBoundingRect()`.
- `GameRenderer`:
  - Thread `TransformHandler` into `FxLayer` construction.


## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [ ] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [ ] I have added relevant tests to the test directory
- [ ] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

DISCORD_USERNAME

src/client/graphics/GameRenderer.ts

@@ -269,7 +269,7 @@ export function createRenderer(
     structureLayer,
     samRadiusLayer,
     new UnitLayer(game, eventBus, transformHandler),
-    new FxLayer(game),
+    new FxLayer(game, transformHandler),
     new UILayer(game, eventBus, transformHandler),
     new NukeTrajectoryPreviewLayer(game, eventBus, transformHandler, uiState),
     new StructureIconsLayer(game, eventBus, uiState, transformHandler),