This adds a special VFS providing textures/radiant/notex and shadernotex.
This way, game packs does not have to provide them using the hardcoded
textures/radiant/[shader]notex path (some games prefer other prefixes).
It also allows GtkRadiant to not crash at startup when a wrong game
path is set.
It would also prevents radiant to fail where it tries to load these
textures on very early stage when the whole VFS is not yet loaded,
for example on pk3dir scenario.
The notex and shadernotex textures come from Unvanquished's tex-common
package and are distributed under public domain-like CC0 1.0 Universal license: https://creativecommons.org/publicdomain/zero/1.0/
See: https://github.com/UnvanquishedAssets/tex-common_src.dpkdir/blob/master/about/tex-common.txt
Thomas Debesse