Cache: Dispose of ImageBitmap data when possible

[gkjohnson]

ImageBitmap includes a close function which allows for immediately discarding of image data when it's no longer needed. We could at least call this on disposeTile if not sooner. It should not be called earlier than first render, though, because it cannot be closed before upload to GPU. If the renderer is being used in multiple WebGLRenderers then it shouldn't ever be discarded until the tile is disposed.