Online pack sprites from loader

[lordmauve]

wasabi2d includes an image loader that it inherits from Pygame Zero for loading images from the images/ directory. However this is not currently used in texture packing.

Currently rectpack's offline mode is used and we pass a hard-coded list of images to pack. Even pre-packing all the images in the images directory would be better than this. However, online packing the images that actually get requested is a little more scalable. Sadly rectpack's online solver does not return the location to pack the next image - an issue that is relatively but not completely shallow - see rectpack#27.

We must no longer convert the alpha channel (does this premultiply)?:
https://github.com/lordmauve/wasabi2d/blob/master/wasabi2d/loaders.py#L183

[lordmauve]

Done - by replacing rectpack with my own implementation.