Feature/asset loader

[jarcane]

Adds a new key function and API for loading assets from URLs with proper loading confirmation before to start the game.

At present, this doesn't work, for two reasons:

• loading never terminates because the atom containing the load counts never updates. The callback is called, because the log message triggers, but the call to swap! doesn't seem to happen at all.
• the asset-loader function for some reason returns the integer request id from requestAnimationFrame instead of returning the asset map

Posting this as WIP 'cause I could use some eyes on.

[jarcane]

Further investigation by adding a watcher confirms that actually the swap! call is completed:

loaded https://images.pexels.com/photos/957010/milky-way-starry-sky-night-sky-star-957010.jpeg 
"-- Atom Changed --" 
"key" :watcher 
"atom" #object[cljs.core.Atom {:val {:loaded 1, :error 0, :total 1}}] 
"old-state" {:loaded 0, :error 0, :total 1}
"new-state" {:loaded 1, :error 0, :total 1} 

However, for some reason when we deref the value, it still shows the old-state. Something is happening out of order here somehow, and I've no idea why.

[jarcane]

Continuing investigation: the atom is not the problem. Swaps are happening just fine.

The problem is that instead of returning assets as it should, the return value of asset-loader continues to be the return value of requestAnimationFrame, even when the recursive call should terminate and return the value from the if statement instead. This then causes a crash, because then the demo tries to use that number as an asset map and obviously fails.