Implement UnhandledExceptionFilter and revert "set intro scene background to white"

Also use g_iScreenWidth and g_iScreenHeight in RMLManager

Minecraft.Client/Common/UI/UIScene_MessageBox.cpp

@@ -143,10 +143,6 @@ void UIScene_MessageBox::reloadMovie(bool force)
 {
 }
 
-//GabsPuN: When Iggy is completely replaced, some functions should be moved to UIScene.cpp.
-//This way, the tick() function won't be repeated in every UIScene_ cpp file.
-//Also, since iggy no longer exists, we can take liberties with optimization or code
-//that is easier to work with.
 void UIScene_MessageBox::tick()
 {
 	m_hasTickedOnce = true;