Texture atlas, bitmap fonts, fix texture caching

* A TextureAtlas class was added. What this allows you to do is having
an image with a grid of textures inside, reducing file amount and making
a lot of things easier.
* Following said texture atlas, bitmap fonts were also added. A sample
fontsheet is available.
* Texture caching is now enabled by default again.
* The default font renderer for new components can be set (the field is
not final anymore).