Change indexes into tile arrays to be stored as bytes to save memory.

The index value needs only be big enough to handle all defined terrain types. This is a low number so we can save memory by defining it as a byte. This particularly saves memory for the CustomTerrain field in the Map class, which defines a cell layer for the map using tile indexes, so we can reduce the size of that layer 4x as a result.
Don't store pre-calculated texture coords in Sprite.

By storing only the four corners, we can save the object overhead of an array and 4 float elements per sprite. This results in savings of around 5 MiB to store these coordinates.
HashSet<byte> => BitArray resourceTypeIndices

BitArray can store 256 bits in 32 bytes plus overhead (pointer & length),
which gives it better memory locality too (<= 2 locations in memory).
Also, the actual count will be much less, probably at most around a dozen.
This might not impact performance, but the AI is dumb & HashSet<byte> is
clearly inefficient.