gg: add window_size() function (#8409)

vlib/gg/gg.v

@@ -660,6 +660,11 @@ pub fn screen_size() Size {
 	return Size{}
 }
 
+// window_size returns the `Size` of the active window
+pub fn window_size() Size {
+	return Size{sapp.width(), sapp.height()}
+}
+
 fn C.WaitMessage()
 
 /*