function _getCache( renderer ) {

	let rendererCache = _cache.get( renderer );

	if ( rendererCache === undefined ) {

		rendererCache = new WeakMap();
		_cache.set( renderer, rendererCache );

	}

	return rendererCache;

}
Returns a cache that stores generated PMREMs for the respective textures.
A cache must be maintained per renderer since PMREMs are render target textures
which can't be shared across render contexts.

@private
@param {Renderer} renderer - The renderer.
@return {WeakMap<Texture, Texture>} The PMREM cache.