Script API: GUIMouseUp and Down events have click position as relative

There are two considerations for this:
- The events refer to a mouse event over a particular GUI, not all or any gui, so it does not make much sense to have pos in screen coordinates.
- In theory, gui's position may also change between recording a mouse event in the engine and script function call.

Engine/ac/gui.cpp

@@ -774,7 +774,7 @@ void gui_on_mouse_up(const int wasongui, const int wasbutdown, const int mx, con
         break;
     }
 
-    run_on_event(kScriptEvent_GUIMouseUp, wasongui, wasbutdown, mx, my);
+    run_on_event(kScriptEvent_GUIMouseUp, wasongui, wasbutdown, mx - guis[wasongui].X, my - guis[wasongui].Y);
 }
 
 void gui_on_mouse_down(const int guin, const int mbut, const int mx, const int my)
@@ -785,7 +785,7 @@ void gui_on_mouse_down(const int guin, const int mbut, const int mx, const int m
     if ((guis[guin].MouseDownCtrl < 0) && (!guis[guin].OnClickHandler.IsEmpty()))
         force_event(AGSEvent_GUI(guin, -1, static_cast<eAGSMouseButton>(mbut)));
 
-    run_on_event(kScriptEvent_GUIMouseDown, guin, mbut, mx, my);
+    run_on_event(kScriptEvent_GUIMouseDown, guin, mbut, mx - guis[guin].X, my - guis[guin].Y);
 }
 
 //=============================================================================