prevent crashes when unexpectedly forced to exit fullscreen mode

(e.g. using windows keys in fullscreen)


Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@2251

src/hardware/vga_draw.cpp

@@ -523,3 +523,7 @@ void VGA_SetupDrawing(Bitu val) {
 		PIC_AddEvent(VGA_VerticalTimer,vga.draw.delay.vtotal);
 	}
 };
+
+void VGA_KillDrawing(void) {
+	PIC_RemoveEvents(VGA_DrawPart);
+}