Fix missing elements in some older saves not showing up as such

Broken since a38e1c48bb, where older saves were allowed to work off the legacy identity mapping of save numbers to element numbers. The problem was that this could leave even those element numbers mapped that were associated by the palette with unknown element identifiers. These elements would then fail to spawn but would not show up as missing in the save preview.

Still not perfect, see the TODO in the diff.

src/client/GameSave.cpp

@@ -52,6 +52,24 @@ GameSave::GameSave(const std::vector<char> &data, bool newWantAuthors)
 
 void GameSave::MapPalette()
 {
+	// - the palette is always right
+	//   - there are palettes with missing entries but there are no palettes with incorrect entries
+	//   - for every (identifier, number) pair in the palette
+	//     - if the identifier is recognized, map the number to the appropriate element
+	//     - if not, map it to 0
+	//       - complain about the identifier if the corresponding number is actually used
+	// - to handle every number not covered by the palette
+	//   - in the case of any 98.0+ save, the palette is comprehensive
+	//     - except for the few cases already handled below
+	//       - e.g RSSS et al not having CarriesTypeIn set properly until 98.2
+	//       - can handle mistakes like this with similar extra code later
+	//     - map any number seen used to 0 and complain about the number
+	//   - in the case of any pre-98.0 save, the palette may not be comprehensive
+	//     - do the legacy identity mapping
+	//       - this is not perfect because it lets numbers slip that are clearly invalid
+	//       - TODO: maybe identity-map only ranges of numbers that are known to have existed
+	//         at the point in time in vanilla when the save was made, based on this->version
+
 	int partMap[PT_NUM];
 	bool ignoreMissingErrors[PT_NUM];
 	for(int i = 0; i < PT_NUM; i++)
@@ -91,11 +109,8 @@ void GameSave::MapPalette()
 						myId = i;
 					}
 				}
-				if (myId)
+				partMap[pi.second] = myId;
-				{
+				if (!myId)
-					partMap[pi.second] = myId;
-				}
-				else
 				{
 					missingElementIdentifiers.insert(pi);
 				}