- Updated directional_light.prim. Now it includes the mesh's normals.
- Added UNTESTED code so now Meshes include polygons groups, each one with an associated material (added support for multi-material meshes).
	- By extension, also primitives and primitives importers include polygon groups and multiple materials.
- Minor fixes and couts removal.
- Packet class now contains two inner buffers, one for the packet's header and another for the packet's body. The latter is dynamically allocated.