Add cancel to ResourceManager

[mnordine]

No description provided.

[AndrewLugg]

I have put some work in on this feature, it will currently be able to cancel assets that are downloaded using the imageBitmap feature as it uses a HTTP request, I haven't got the cancel working on ImageElement as Image elements are trickier to cancel.
I got it working with Texture Atlas's as well.

Currently doesn't do anything with other assets such as audio.

It will also add progress for images, which will return a percentage completed of the image if it supports it.

https://github.com/bp74/StageXL/tree/cancellable-assets

[mnordine]

@AndrewLugg I have this working as well, you can see some commits here: https://github.com/mnordine/StageXL/commits/cancellation

9d87653
a6590c7
a945fdc
e66e200
ed69280
639398e
ec4c48f
ae79f6a