Make sure that no key is being marked down

It's apparently not enough to clear key repeats, we'll need to clear
the down states too. Without this Alt stays pressed after toggeling
fullscreen trhough Alt-Enter.

src/backends/generic/vid.c

@@ -195,7 +195,6 @@ VID_CheckChanges(void)
 		S_StopAllSounds();
 
 		/* refresh has changed */
-		vid_fullscreen->modified = true;
 		cl.refresh_prepped = false;
 		cl.cinematicpalette_active = false;
 		cls.disable_screen = true;

src/client/cl_keyboard.c

@@ -1149,5 +1149,6 @@ Key_MarkAllUp(void)
 	for (key = 0; key < K_LAST; key++)
 	{
 		key_repeats[key] = 0;
+		keydown[key] = 0;
 	}
 }