Add 2x and 4x Balance Mod effects packs

 * The code now always tries to load balance-2x and balance-4x when
   it would load hires2x/hires4x.

 * The 2x and 4x effects packs have rescaled low-res graphics right now.
Update uqmpack.nfo files for the new addons
Effects pack fixes, fix crash on startup

 * The game segfaulted on startup before, this fixes it.

 * The RMPs in the new addon packs are now fixed.

 * The addons will no be loaded in such an order that the
   hires packs will overwrite the lowres one.