TextureTools: initial rude implementation of distance field computation.

Not yet ported anywhere else than GL >= 3.3 with some GL 4.3 features.
Still, this closes #3.
TextureTools: proper function name in assert.
A little bit of const fascism.
TextureTools: support padding in atlas().
Text: minor code cleanup.
Text: prerendering Font for use with distance-field rendering.

Currently there is no builtin shader capable of it, will come later.
Shortened (texture) filtering types to comply with OpenGL naming.

The documentation already used them, so why complicate things further.