Implement more rendering optimisations

[Inspirateur]

These optimisations should provide another big performance boost to the rendering (cf: this excellent video on the topic)

• [x] 1 chunk mesh per face type, culled to only display the 3 visible faces to the player
• [ ] instancing & batching

[Inspirateur]

So I just tried implementing the first optimization in the list (with the help of @kokounet !) BUT frame rate took a big hit (approx x3 times worse).

After checking with RenderDoc, the performance issue seems to be caused by draw calls (~100k per frame, way too much!) This is because we're doing 1 draw call per mesh so multiplying the meshes by 6 (1 per face type) and culling half of them ~multiplies draw calls by 3.

In conclusion the first optimization will only pay out when we manage to batch draw calls, which is next on the list.