Cache VoxelGrid meshing results (RenderMesh)

[RiscadoA]

Problem

Currently we always mesh voxel grid assets which are stored in the disk.
When shipping a game, this doesn't seem to make much sense, as it could have been already distributed as a mesh, instead of a voxel grid (or both).
In big samples/demos, this actually leads to a very slow startup, as meshing is a quite expensive algorithm.

Solution

With #1413, it would be trivial to solve this problem. We could simply change the RenderMeshBridge to check if there's a .mesh file, and use if it so. The save method could write the tesselated mesh into the .mesh file. Thus, when deploying games, if we want faster loads, we could simply save all render meshes into the assets directory by calling Assets::save on them.