Merge branch 'matthiaskrgr/afterslime_windowfix' into 'master'

afterslime: fix window when viewing from inside slime (area near mortar)

before:
front: (ok) ![xonotic20160325030620-00](/uploads/f132cc6b12ea7a738cb001382dda53e6/xonotic20160325030620-00.png)

back before ![xonotic20160325030639-00](/uploads/a65a20f2aadb320ce6a913396f96bdd5/xonotic20160325030639-00.png)

(you can see through the sides which are solid viewed from the front)

back fixed: ![xonotic20160325031231-00](/uploads/e3d0d010d5043200eef35a9da2d76aff/xonotic20160325031231-00.png)

See merge request !51