screenx and screeny should only consider width and height

On a multi head display, screen can be positioned on e.g. x pos 1000 and have
width of 1000 which makes screenx 0. This makes defx negative.

src/myframe.h

@@ -679,8 +679,8 @@ struct MyFrame : wxFrame {
 
         int display_id = wxDisplay::GetFromWindow(this);
         wxRect disprect = wxDisplay(display_id == wxNOT_FOUND ? 0 : display_id).GetClientArea();
-        const int screenx = disprect.width - disprect.x;
-        const int screeny = disprect.height - disprect.y;
+        const int screenx = disprect.width;
+        const int screeny = disprect.height;
 
         const int boundary = 64;
         const int defx = screenx - 2 * boundary;