gallium: split transfer_{map,unmap} into {buffer,texture}_{map,unmap}, remove u_resource_vtbl

This removes the call indirection via u_resource_vtbl, and thus removes the vtbl pointer from all pipe_resources.

resource_destroy and transfer_flush_region are not changed, but must check whether the resource target is a buffer or texture.

transfer_map and transfer_unmap are split into buffer and texture callbacks.