Fix local player pointer randomly becoming corrupted after spawning an item

[yyy257]

Client-side entitites are tracked with an M*LIB dictionary, and inserting a new entity can shuffle other entities around in memory, including the local player:

Elements in the dictionary are unordered. On insertion of a new element, contained objects may moved.

Therefore, insertion of a new entity can randomly make gstate.local_player point to garbage data, usually getting the player stuck at X=0, Y=0 and Z=0, sometimes ending in a segmentation fault.

To fix it, I added a variable separate from the entity dictionary which tracks the local player ID, allowing updating gstate.local_player after spawning a new entity. As long as the game is singleplayer, the local player ID is always 0, but the variable may be useful when multiplayer is added.

[xtreme8000]

Good spot! I think the best solution here would be to only store struct entity* pointers in the dict, instead of the whole object.

[yyy257]

I also think that's a good idea, so I implemented it in a commit. It might not be the best implementation, but it seems to work - I hope it will be useful. This could even save memory, as dictionaries may use a temp array relative to the dictionary's size while resizing.

To reproduce the bug, I dug a lot of blocks without picking them up until the player data became corrupted. I could also reliably trigger it with the code in my fork that drops all items upon death.