Generate atmospheres for gas giants in legacy custom systems

- Gas giants previously depended on a fixed 14.0 atmospheric density
- Add an import step which generates a plausible atmospheric density if a gas giant defined in Lua doesn't have its atmosphere set.

src/galaxy/CustomSystem.cpp

@@ -1018,7 +1018,9 @@ void CustomSystemsDatabase::RunLuaSystemSanityChecks(CustomSystem *csys)
 
 		bool wantRings = body->ringStatus == CustomSystemBody::WANT_RANDOM_RINGS || body->ringStatus == CustomSystemBody::WANT_RINGS;
 
-		if (!(body->want_rand_offset || body->want_rand_phase || body->want_rand_arg_periapsis || wantRings))
+		bool wantAtm = body->bodyData.m_type == SystemBodyType::TYPE_PLANET_GAS_GIANT && body->bodyData.m_volatileGas == fixed(0);
+
+		if (!(body->want_rand_offset || body->want_rand_phase || body->want_rand_arg_periapsis || wantRings || wantAtm))
 			continue;
 
 		// Generate body orbit parameters from its seed
@@ -1047,6 +1049,12 @@ void CustomSystemsDatabase::RunLuaSystemSanityChecks(CustomSystem *csys)
 			}
 		}
 
+		if (wantAtm) {
+			// Taken from StarSystemRandomGenerator::PickPlanetType
+			body->bodyData.m_volatileGas = rand.NormFixed(fixed(1050, 1000), fixed(8000, 1000)).Abs();
+			body->bodyData.m_atmosOxidizing = rand.NormFixed(fixed(0, 1), fixed(300, 1000)).Abs();
+		}
+
 	}
 }