Move returning to editor to end of frame

This fixes horizontal and vertical warp backgrounds not resetting, and
also a bunch of other 1-frame glitches, most noticeably cutscene bars
and fadeouts.

This adds a new variable shouldreturntoeditor to Game to signal whether
or not it should return to editor at the end of the frame.

desktop_version/src/Game.h

@@ -388,6 +388,7 @@ public:
 
 #if !defined(NO_CUSTOM_LEVELS)
     void returntoeditor();
+    bool shouldreturntoeditor;
 #endif
 };