Handle writing to unloaded blocks in MMVManip::blitBackAll()

This could happen before and would just silently discard the data,
but now that Lua can create VManips without loading from map beforehand
we definitely need to handle this case.

src/voxel.cpp

@@ -15,7 +15,6 @@
 	Debug stuff
 */
 u64 emerge_time = 0;
-u64 emerge_load_time = 0;
 
 VoxelManipulator::~VoxelManipulator()
 {