Merge branch 'maddin/textures-common' into 'master'

Maddin/textures common

This branch contains the following fixes and improvements:
* It adds the previously missing textures `common/botclip` and `common/noimpact`.
* All common-textures have been recreated with a uniform look and font (using Xolonium).
* Extremely reduced file sizes. Factor is about 5 in average.
* No compression ugliness visible anymore.
* 3 completely new base-shaders which will be useful for mappers blocking out their maps (so you can focus on the structure and do actual texturing later).
* Fixing typo in `common/antiportal` and `common/nodrawnonsolid`.
* Every texture now has a clear name matching the shader.

Here is a preview of the new textures:
![preview](https://gitlab.com/xonotic/xonotic-maps.pk3dir/uploads/5ebe217097a0c01e65223c3161acc024/preview.png)

See merge request !47