Getting textures to work was a bit tricky, since we basically have no
more texture IDs in modern shaders (they are mere nodes).

Modified specular conversion to be quadratic (between FBX Phong exponent
to Pricipled specular factor).

Also fixed several issues in both importers and exporters.

And cleaned up ugly usage of 'mat' short name for materials in exporter
(mat is reserved for matrix in Blneder code in general, 'ma' is short
for material).