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feat: Materials can now use the screen texture (#2849)


https://github.com/excaliburjs/Excalibur/assets/612071/ade92ec3-caff-4835-8410-74f4ebbbe421

This PR adds a new feature for materials that allows them to reference the screen texture. They way it works is it's the screen's framebuffer RIGHT before the material draw call (if you include the complete drawing you get the infinite mirror artifact which is pretty unusable)

2 new uniforms

* `u_screen_texture` - This is the texture of the screen right before the material draw call
* `u_time_ms` - This is the milliseconds since page navigation (`performance.now()` under the hood)

2 new attribute/varyings

* `a_screenuv` - The vertex attribute corresponding to the screen uv relative to the current graphic
* `v_screenuv` - The fragment varying corresponding to the screen uv relative to the current graphic

Finally there is a new convenience api for updating shader values in materials. `.update(shader => {...})`

```typescript
  game.input.pointers.primary.on('move', evt => {
    heartActor.pos = evt.worldPos;
    swirlMaterial.update(shader => {
      shader.trySetUniformFloatVector('iMouse', evt.worldPos);
    });
  });
```

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