gmenu: return an indication of handled keys from gmenu_presskeys()

This patch updates the function `gmenu_presskeys()` to return `true`
if the keys are actually handled. This allows the caller function
`PressKey()` to continue processing keys and not silently drop them,
even when the game menu is active.

Why bother? First of all, there should not be any reasons not to
handle keys once the game menu is open. For example, why not handle
the "Quick Load" shortcut once the player is dead and the menu is
shown on the screen?

The other reason is more practical. In headless mode, once the game is
controlled by a 3rd party application (training of an AI agent, for
example), lost key presses lead to a broken training procedure.
Therefore, even if the menu is raised, it is crucial not to lose a
keystroke.

Signed-off-by: Roman Penyaev <[email protected]>

Author: rouming

Source/gmenu.cpp

@@ -299,8 +299,10 @@ bool gmenu_presskeys(SDL_Keycode vkey)
 		GmenuUpDown(true);
 		break;
 	default:
-		break;
+		// Key was not handled by the gmenu
+		return false;
 	}
+	// Key was handled
 	return true;
 }