Geometry order now defined by material slot order

Material export now binds one-to-one with blender materials (i.e.
show_double_sided from the blender mesh is no longer used).  This flag
has been moved to materials as "double_sided" (if only because I
couldn't work out what to do when you have two meshes using the same
material, with one being double sided and the other not).

Shadeless materials have also been added (they use the Unlit
techniques). NOTE: specularity is ignored if shadeless is true.

We now create a geometry per material slot (even if that material isn't
used) - this is to give us predictable behaviour in Urho (slot 1 in
blender == slot 1 in Urho).
Zero material error fixed

The last change linking geometry to material instances meant that 0
materials broke the export.  This has been rectified so that a minimum
of 1 geometry is ALWAYS created.
Sort vertex weights during decomposition

Vertex influences are now sorted (and capped to 4) during decomposition.