Public AssetListLoader & examples update

[ellthompson]

This PR brings back the AssetListLoader class which allows an array of assets to be loaded before the 'ready' callback is called. This class is made public and includes unit tests.

The examples browser now makes use of the AssetListLoader for asset loading in all examples. The examples browser therefore no longer needs to hoist the application instantiation in each example in order to preload example assets in.

New Public API:

// Create a new AssetListLoader using a list of assets to load and the asset registry used to load and manage them.
pc.AssetListLoader.constructor(assetList: pc.Asset[], assetRegistry: pc.AssetRegistry): void

// Destroys the AssetListLoader.
pc.AssetListLoader.destroy(): void

// Start loading asset list, call done() when all assets have loaded or failed to load.
pc.AssetListLoader.load(done: Function, scope: ?object): void

// Sets a callback which will be called when all assets in the list have been loaded.
pc.AssetListLoader.ready(done: Function, scope: ?object): void

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.

[willeastcott]

Looks good to me, in general. I've left a bunch of comments. However, should we take a moment to review the AssetListLoader API? Do we like the naming? Is the public interface properly established? Do the function signatures make sense (e.g. the constructor taking the asset registry as the second param instead of the first)? Should this functionality just be a function on the AssetRegistry directly instead?

[Maksims]

Would it be possible to write a new API in similar format in the first post of this PR?

[ellthompson]

Would it be possible to write a new API in similar format in the first post of this PR?

Sure, I've added this in.

[mvaligursky]

AssetListLoader.load(done: Function, scope: ?object): void
AssetListLoader.ready(done: Function, scope: ?object): void

I just wonder if the ready function is needed? It does not seem to expose anything additional to the load function. Is this being used by the examples?

[ellthompson]

AssetListLoader.load(done: Function, scope: ?object): void
AssetListLoader.ready(done: Function, scope: ?object): void

I just wonder if the ready function is needed? It does not seem to expose anything additional to the load function. Is this being used by the examples?

It's not used by the examples but it allows other parts of an application to subscribe to the loading of these assets without triggering the load itself.

[ellthompson]

I'm not sure it works like that .. all browsers apart from IE11 implement the function, and that is accessible in ES5 as well .. we don't need a polyfill for ES5, but for IE11. Perhaps check ES5 version of the engine built - i likely just calls this function directly.

Yeah you're right, i've included this in the list of example browser polyfills.

[mvaligursky]

approving, nicely done