Fix fake null-motion mouse event flood

This commit also improves a bit the code quality by making the intent of fake events (and themselves) more explicit.

Fixes #26460.

core/os/input_event.cpp

@@ -33,6 +33,9 @@
 #include "core/input_map.h"
 #include "core/os/keyboard.h"
 
+const int InputEvent::DEVICE_ID_TOUCH_MOUSE = -1;
+const int InputEvent::DEVICE_ID_INTERNAL = -2;
+
 void InputEvent::set_device(int p_device) {
 	device = p_device;
 }