Restructured mesh classes a bit.

Now there is the basic MeshInterface (just Format, buffer type, vCount and prim type).
Then MeshBase extends MeshInterface with a single VBO id and bindings cache. Then Mesh and MeshIndexed extend MeshBase to provide differents APIs to that single VBO.
Next step: implement a class that extends MeshInterface that uses one VBO/format, and one draw call/format,shader. More info: glMultiDrawArraysIndirect (multi draw + instancing).