Substantial MaterialSearcher performance improvement for importing maps

[vesuvian]

The MaterialSearcher was getting very slow if there were a lot of cache misses or a lot of materials in the map.

I've reworked it so it caches the available materials in a single pass, then works against the cache. The result is substantially faster.

[Henry00IS]

I was skeptical at first just how much of an improvement this change would give. The first time I tried this on a full HL2 map (with over 1GB of materials) I had to wait 25 seconds before the import processing began- but (in my chisel version) the level imported so fast that I could barely see the progress bar. Subsequent imports, even after Unity restarts, no longer had that initial delay making it as fast as loading a map in Hammer. Fantastic work @vesuvian! Thank you so much! 😁