Refactor mesh component and resources

[invor]

• move handling of sub-meshes (surface?) from the mesh component manager to the resource manager
• organise free space in a GPU mesh resource using free ranges instead of allocated/used counter
• open question: if sub-mesh is unloaded, how will the mesh component manager know? Especially for procedural meshes? Additional ready/unloaded flag in mesh component? => sub-meshes need a dedicated ready flag, also when GPU mesh resource is ready, but the data of a sub mesh has not been copied yet!