game.shouldreturntoeditor was added to fix a frame ordering issue that
was causing a bug where if you started playtesting in a room with a
horizontal/vertical warp background, and exited playtesting in a
different room that also had a horizontal/vertical warp background and
which was different, then the background of the room you exited in would
slowly scroll offscreen, when you re-entered the editor, instead of the
background consisting entirely of the actual background of the room.
Namely, the issue was that the game would render one more frame of
GAMEMODE after graphics.backgrounddrawn got set to false, and re-set it
to true, thus negating the background redraw, so the editor background
would be incorrect.
With defer callbacks, we can now just use a couple lines of code,
instead of having to add an extra kludge variable and putting handling
for it all over the code.
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