Fixed texture size display when only one texture size is given

Made selected texture size only appear in merge list when there are more than two texture sizes to choose from. The idea is that we should specify texture sizes, even when there is no choice, to provide more information. This may be useful to the user in choosing between mods and we should eventually provide a way for the user to see this information (in search results, and probably in the interface for #52 (closed), though for that we'll likely need to provide texture sizes for each asset group).