This should substantially reduce the number of malloc/free calls made to
create and destroy mobjs, especially during level load and net
save/load.
memory_resource standard header is not available on the OSX deployment
target we are using, so I had to write my own fixed size pool allocator.
- TERRAIN index is +1 (it can write terrainOverlay at the same time, so there is the possibility of needing to archive NULL terrain).
-TERRAIN lookup is done immediately instead of at P_RelinkPointers (dunno why I did this, TERRAIN doesn't do mobjnum crap).
I do not trust memcpy due to the rendering bug even though it appeared to mostly work, so I did it manually. It's still order of magnitudes faster than reloading the level from scratch.
