prepare

Gets ready to resolve the effects in composition, before any of them is asked for.

Every export engine calls this before each negotiation of composition, so work a platform can only do asynchronously, such as decoding an overlay image in a browser, is done by the time resolve needs it. An engine runs one negotiation at a time, so what this reads stays the negotiation's own, and this must not start another negotiation on the engine calling it. The default does nothing.