catharsis: fix some horizont-brushes looking cut off in the bases
before: ![xonotic20150910004727-00](https://gitlab.com/xonotic/xonotic-maps.pk3dir/uploads/
0bb6e937b82d16b4a0fc39ee5020af3e/xonotic20150910004727-00.jpg)
![xonotic20150910004724-00](https://gitlab.com/xonotic/xonotic-maps.pk3dir/uploads/
677a579d27a0da48f7da501de7341324/xonotic20150910004724-00.jpg)
after: ![xonotic20150910005921-00](https://gitlab.com/xonotic/xonotic-maps.pk3dir/uploads/
a543ce292b165c632c83f52eabe54b41/xonotic20150910005921-00.jpg)
![xonotic20150910005919-00](https://gitlab.com/xonotic/xonotic-maps.pk3dir/uploads/
bc47ca7cf7123b6f609744a2cb41acf0/xonotic20150910005919-00.jpg)
See merge request !40