Combine gasmask filters

Repacking allowed.

Makes it possible to combine gasmask filters. Every conversion happens with a 10% loss.

This mod has 2 major advantages compared to vanilla.
- You don't have to carry a huge amount of low % filters in your inventory
- When you combine a filter with another one and the 'donor' filter uses up all quantity it simply gets removed instead of leaving the player with an empty filter that they would drop from their inventory 99% of the time.

Empty filters also add to the total filter count in the world, meaning that the game won't spawn other filters in the place of the dropped empty ones.
With this mod combined empty filters simply get removed and let other filters spawn all around the map.

Also makes using filters much safer + less annoying because you don't have to wait for your filter to get used until it has only 1-2%. You can simply swap filters at let's say 10% and just combine them later.