• Re-rasterise a heat-map colour texture from a new scalar field, mutating the supplied ImageData in place.

    This is the slim sibling of paintHeatMap, intended for live updates: the geometry's UVs already exist (the initial paintHeatMap call wrote them onto SceneGeometry.uvsCompressed), the SceneTexture's pixel buffer already exists, and the normal / MR maps don't change between frames. So this function skips UV-axis selection, UV emission, and normal/MR allocation — everything left is rasterise + dilate + blur(×2) + optional grid, straight into the caller's buffer.

    Pair with texture.imageData = imageData to fire the onSceneTextureImageDataChanged event, which makes the renderer re-upload just the affected atlas sub-rect via texSubImage2D — no batch / mesh / material rebuild.

    Returns void