Add support for loading images

[tuupola]

https://github.com/kikuchan/pngle
http://elm-chan.org/fsw/tjpgd/00index.html

[RobLoach]

Of course, stb image is an option too https://github.com/nothings/stb/blob/master/stb_image.h

[oclyke]

Is the goal here to add compatibility with other libraries or to engulf another library to get built-in support?

I would argue in favor of the first option so as to keep HAGL as platform-independent as possible. Perhaps methods like load_from_pngle() and load_from_stb() and load_from_X() could enable the interface.