Feat/reduce drawcalls

[piellardj]

This PR changes the rendering engine. Compared to the previous version:

• each Patch is now made of a single Mesh (compared to 6 before). This reduces the number of draw calls by 6 and greatly improves performance.
• bumped the max number of voxel materials to 32768
• the downside is that each Patch now takes twice as much GPU memory, but it remains low (< 1 Mo per 64x64 patch)