optimize adding background changes

I have a song with 5841 background changes and it took 3 seconds to load that song.

src/BackgroundUtil.cpp

@@ -129,6 +129,13 @@ void BackgroundUtil::SortBackgroundChangesArray( vector<BackgroundChange> &vBack
 	sort( vBackgroundChanges.begin(), vBackgroundChanges.end(), CompareBackgroundChanges );
 }
 
+void BackgroundUtil::AddBackgroundChange( vector<BackgroundChange> &vBackgroundChanges, BackgroundChange seg )
+{
+	vector<BackgroundChange>::iterator it;
+	it = upper_bound( vBackgroundChanges.begin(), vBackgroundChanges.end(), seg, CompareBackgroundChanges );
+	vBackgroundChanges.insert( it, seg );
+}
+
 void BackgroundUtil::GetBackgroundEffects( const RString &_sName, vector<RString> &vsPathsOut, vector<RString> &vsNamesOut )
 {
 	RString sName = _sName;

src/BackgroundUtil.h

@@ -73,6 +73,7 @@ struct BackgroundChange
 /** @brief Shared background-related routines. */
 namespace BackgroundUtil
 {
+	void AddBackgroundChange( vector<BackgroundChange> &vBackgroundChanges, BackgroundChange seg );
 	void SortBackgroundChangesArray( vector<BackgroundChange> &vBackgroundChanges );
 	
 	void GetBackgroundEffects(	const RString &sName, vector<RString> &vsPathsOut, vector<RString> &vsNamesOut );

src/Song.cpp

@@ -175,14 +175,12 @@ void Song::AddBackgroundChange( BackgroundLayer iLayer, BackgroundChange seg )
 	}
 
 	ASSERT( iLayer >= 0 && iLayer < NUM_BackgroundLayer );
-	GetBackgroundChanges(iLayer).push_back( seg );
+	BackgroundUtil::AddBackgroundChange( GetBackgroundChanges(iLayer), seg );
-	BackgroundUtil::SortBackgroundChangesArray( GetBackgroundChanges(iLayer) );
 }
 
 void Song::AddForegroundChange( BackgroundChange seg )
 {
-	GetForegroundChanges().push_back( seg );
+	BackgroundUtil::AddBackgroundChange( GetForegroundChanges(), seg );
-	BackgroundUtil::SortBackgroundChangesArray( GetForegroundChanges() );
 }
 
 void Song::AddLyricSegment( LyricSegment seg )