mesh: mesh_buffer: Remove data size fields

Currently, mesh_buffer is persistently mapped immutable storage, so its
size never changes. But for drawing static geometry, this isn't needed.
Additional functionality and some tweaks to existing functionality is
needed to allow mesh_buffer to be used for such data. It might even be
wise to rename the current mesh_buffer to dynamic_mesh_buffer and create
a new one called static_mesh_buffer, where the dynamic and static refer
to the buffer, not the data they hold.