Keep track of GameTDB ID separately from game ID

The difference between Dolphin's game IDs and GameTDB's game IDs
is that GameTDB uses four characters for non-disc titles, whereas
Dolphin uses six characters for all titles.

This fixes:

- TitleDatabase considering Datel discs to be NHL Hitz 2002
- Gecko code downloading not working for discs with IDs starting with P
- Cover downloading mixing up discs with channels (e.g. Mario Kart Wii
  and Mario Kart Channel) and making extra HTTP requests. (Android was
  actually doing a better job at this than DolphinQt!)

Source/Core/Core/IOS/ES/Formats.cpp

@@ -323,6 +323,20 @@ std::string TMDReader::GetGameID() const
   return StringFromFormat("%016" PRIx64, GetTitleId());
 }
 
+std::string TMDReader::GetGameTDBID() const
+{
+  const u8* begin = m_bytes.data() + offsetof(TMDHeader, title_id) + 4;
+  const u8* end = begin + 4;
+
+  const bool all_printable =
+      std::all_of(begin, end, [](char c) { return std::isprint(c, std::locale::classic()); });
+
+  if (all_printable)
+    return std::string(begin, end);
+
+  return StringFromFormat("%016" PRIx64, GetTitleId());
+}
+
 u16 TMDReader::GetNumContents() const
 {
   return Common::swap16(m_bytes.data() + offsetof(TMDHeader, num_contents));

Source/Core/Core/IOS/ES/Formats.h

@@ -205,9 +205,15 @@ public:
 
   // Constructs a 6-character game ID in the format typically used by Dolphin.
   // If the 6-character game ID would contain unprintable characters,
-  // the title ID converted to hexadecimal is returned instead.
+  // the title ID converted to 16 hexadecimal digits is returned instead.
   std::string GetGameID() const;
 
+  // Constructs a 4-character game ID in the format typically used by GameTDB.
+  // If the 4-character game ID would contain unprintable characters,
+  // the title ID converted to 16 hexadecimal digits is returned instead
+  // (a format which GameTDB does not actually use).
+  std::string GetGameTDBID() const;
+
   u16 GetNumContents() const;
   bool GetContent(u16 index, Content* content) const;
   std::vector<Content> GetContents() const;