Add scene-color-map to pc-camera

[willeastcott]

Description

Refractive materials sample the camera's scene color map: a copy of what the camera has drawn before its transparent objects. glTF materials with transmission are among them. Nothing in PWC could ask a camera to render that copy. Debug builds report this in the Car Configurator (the lights lens material) and Vehicle Physics (Warthog_Glass_Mat):

A uSceneColorMap texture is used by the shader but a scene color texture is not available. Use CameraComponent.requestSceneColorMap / enable Color Grabpass on the Camera Component / CameraFrame.rendering.sceneColorMap to enable it.

<pc-camera> now exposes it as the scene-color-map attribute and sceneColorMap property. Both are backed by CameraComponent#renderSceneColorMap, a camera component data property; the Editor's camera exposes it as its color grab pass. The engine's setter holds a single request per component however often it is written, so the element passes the value through like its other properties. It defaults to false, like the engine, because the copy costs a full-screen copy every frame. The name matches the cameraFrame script's rendering.sceneColorMap, which is what a camera rendered through that script uses instead.

<pc-camera scene-color-map></pc-camera>

The examples are unchanged

Turning the attribute on in the two examples removes the debug error, but I measured no visible difference. In same-frame captures with a virtual clock and seeded Math.random, where repeated runs matched pixel for pixel, the mean difference was:

View	Mean difference
Car Configurator's default view	0.00
Vehicle Physics' chase view	0.00
Warthog windscreen, seen head-on	0.01

Behind each of these glass surfaces there is little for refraction to change. A per-frame copy with no visible effect is not worth adding to either page. The attribute is there now if a scene needs it.

Testing

• A scene-color-map row joins the <pc-camera> attribute table. It checks the engine default, setting the attribute in markup, changing it live, and restoring the default on removal.
• A new test checks the engine's request count: 1 at creation, still 1 after repeated writes, and 0 once the attribute is removed.
• npm test, npm run lint, npm run type-check and npm run docs pass. After npm run build, custom-elements.json lists scene-color-map with default false.
• With the attribute set locally on both examples, each camera held exactly one request and had its color grab pass, and the debug engine's uSceneColorMap error was gone.

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