Use window manager for full screen display and implement toggleFullScreen method
Add missing include file
Move fullscreen toggle handling

Move fullscreen toggle handling to the sames places as in the Windows implementation

Followup to 143d40a
Don't stop updating the screen if the mouse is not moved
Fix segmentation faults on exit

Make sure the threads have joined the main thread before terminating the
system thread and the program - otherwise threads may access ressources
already deleted by other threads.
Implement simulation of ESC key

Following the logic of the Windows version simulate an ESC key press
event when the user closes the window.
Some improvements.

1) DGEpsilon was added to avoid the loss of precision.
2) The cycle bug is fixed:
for (int i = 0; i <= DGMaxHotKeys; i++)
_hotkeyData[i].enabled = false;
The condition i <= DGMaxHotKeys is changed for i < DGMaxHotKeys, because
array overrun is possible; the value of 'i' index could reach 13.
Geometry functions.
Merge pull request #1 from Alexander-Alekseev/cthulhu

Cthulhu
Fix warning (var assigned to itself)
Fix warning
Merge remote-tracking branch 'remotes/origin/master' into laenion

Merge with upstream to resolve merge conflict

Conflicts:
	Dagon/DGSystemUnix.cpp
Implement handling of function keys (F1..F12)
Merge branch 'master' of git://github.com/Alexander-Alekseev/Dagon
Merge branch 'patch-2' of git://github.com/AMDmi3/Dagon

Conflicts:
	Other Sources/stb_image.c
Merge branch 'patch-3' of git://github.com/AMDmi3/Dagon