Added post-update event.

This might be too hacky to live but renders have to
happen after updates. I can imagine a lot of uses
for knowing the order of ticks... maybe this is
okay?
Give components the game instance on init.
Iterating over entities by component.

Let it be known that this is when I started to get
tired and made possibly questionable choices.
Rejecting startup task in image onerror.
2d-canvas-renderer beginnings.