filter non-shader texture paths at the same place

The code to filter out useless textures (gloss map, height map, normal map etc.) was spread across two different files, now it's done at the same place.

This commit also add some well-known suffixes to filter out, like the one used for preview images (_p) or the one used for explicit diffuse map (_d) that does not have to be loaded since a shader for them is expected. The _n normalmap alternative is also added.