Display bug when entering game menu screen during a transition.

[Joselle]

If you enter the game menu screen (with right-click or Esc) in the middle of a transition (like a dissolve), and then you exit the game menu screen, the game will be "stuck" with the dialogue box not shown on the screen. The game won't proceed unless you do one of the following:
- Use Ctrl or Tab to start skipping.
- Hide the dialogue window with the H key, and then unhide it.
- Try to close the Ren'Py window and then choose No.

This effect can be reproduced consistently. The template game attached to the following forum post demonstrates the bug:
http://lemmasoft.renai.us/forums/viewtopic.php?f=8&t=10891
The background-color transition is 2 seconds long to make it easy to reproduce the problem, but the transition can be much shorter (0.5 or even 0.1 of a second) and the problem can still happen, if the player has good timing.

The effect happens in Ren'Py 6.12.1.1520 and 6.12.0e. As far as I can tell, it doesn't happen in 6.10.2.

The effect is somewhat likely to be discovered by players: in the game I'm working on, about half of the beta testers observed the effect at least once. (The game is currently about 100,000 words long)

[Tom Rothamel (renpytom)]

This will be fixed in 6.12.2.