Fix touch input on Linux

The code relied on touch IDs being consecutive. This is true on Android, but not on Linux.
Therefore, touch input on Linux was broken since 53886dcdb52de80d862539e22950c84fbf88df88.

src/gui/modalMenu.cpp

@@ -273,7 +273,7 @@ bool GUIModalMenu::preprocessEvent(const SEvent &event)
 		irr_ptr<GUIModalMenu> holder;
 		holder.grab(this); // keep this alive until return (it might be dropped downstream [?])
 
-		if (event.TouchInput.ID == 0) {
+		if (event.TouchInput.touchedCount == 1) {
 			if (event.TouchInput.Event == ETIE_PRESSED_DOWN || event.TouchInput.Event == ETIE_MOVED)
 				m_pointer = v2s32(event.TouchInput.X, event.TouchInput.Y);
 			gui::IGUIElement *hovered = Environment->getRootGUIElement()->getElementFromPoint(core::position2d<s32>(m_pointer));
@@ -290,7 +290,7 @@ bool GUIModalMenu::preprocessEvent(const SEvent &event)
 			if (event.TouchInput.Event == ETIE_LEFT_UP)
 				leave();
 			return ret;
-		} else if (event.TouchInput.ID == 1) {
+		} else if (event.TouchInput.touchedCount == 2) {
 			if (event.TouchInput.Event != ETIE_LEFT_UP)
 				return true; // ignore
 			auto focused = Environment->getFocus();