Buffer item set look-up results to improve performance.

At least on a mid-range mobile device, the item set interface slows the game to roughly 2-3 fps. Since this appears to depend on the number of gems in the set, I’m guessing each gem gets searched every frame. The slow-down is bad enough to make input unresponsive. If the strings to be drawn for each element in the set were stored into a buffer array after the first lookup when the item set is highlighted, it would alleviate this issue.