Write save games to a memory stream first

Two motivations for doing this:

- If the user chooses to overwrite existing save file, and there is an exception during the save process, the existing file will not be lost.
- Many small writes to a file are slow. Very slow. Writing to memory first then writing the completed file to disk appears to be ~500% faster.

apps/openmw/mwstate/statemanagerimp.cpp

@@ -207,7 +207,9 @@ void MWState::StateManager::saveGame (const std::string& description, const Slot
         else
             slot = getCurrentCharacter()->updateSlot (slot, profile);
 
-        boost::filesystem::ofstream stream (slot->mPath, std::ios::binary);
+        // Write to a memory stream first. If there is an exception during the save process, we don't want to trash the
+        // existing save file we are overwriting.
+        std::stringstream stream;
 
         ESM::ESMWriter writer;
 
@@ -262,7 +264,14 @@ void MWState::StateManager::saveGame (const std::string& description, const Slot
         writer.close();
 
         if (stream.fail())
-            throw std::runtime_error("Write operation failed");
+            throw std::runtime_error("Write operation failed (memory stream)");
+
+        // All good, write to file
+        boost::filesystem::ofstream filestream (slot->mPath, std::ios::binary);
+        filestream << stream.rdbuf();
+
+        if (filestream.fail())
+            throw std::runtime_error("Write operation failed (file stream)");
 
         Settings::Manager::setString ("character", "Saves",
             slot->mPath.parent_path().filename().string());