A bunch of more bits for planet data.

The original code is miserly with bits for planet data, which
made sense for a game for that generation of hardware.  I'm
fine with dozens of KB per planet...  Even with tens of thousands
of planets; modern systems won't even be stressed by the memory
usage.  A few hundred megabytes to a gigabyte are available easily
on most platforms that I care about.

sc2/src/uqm/planets/planets.h

@@ -110,7 +110,9 @@ struct planet_desc
 {
 	DWORD rand_seed;
 
-	BYTE data_index;
+	// Expanded to 32 bits now, to permit additional flags
+	// like shield types and such.
+	DWORD data_index;
 	BYTE NumPlanets;
 	SIZE radius;
 	POINT location;