Another atempt to make exported FBX files loadable by FbxViewer: add support for by-polygons material index
(add LayerElementMaterial to mesh data).

This also needed to move connections creations to pre-process func, so that we know FBX-index of each material
for each mesh (assuming indices in LayerElementMaterial refers to order of 'mat to model' connections, which
seems most logical option, but not sure at all this is actually the case).

Anyway, still no luck, next step is to add a SceneInfo element and see whether this works better... :/
More compatibility tries: add 'SceneInfo' element, still not working.