Fixed VS warning.

3rdparty/ocornut-imgui/imgui.cpp

@@ -1740,7 +1740,7 @@ void ImGui::NewFrame()
     // Are we using inputs? Tell user so they can capture/discard the inputs away from the rest of their application.
     // When clicking outside of a window we assume the click is owned by the application and won't request capture.
     int mouse_earliest_button_down = -1;
-    for (size_t i = 0; i < IM_ARRAYSIZE(g.IO.MouseDown); i++)
+    for (int i = 0; i < IM_ARRAYSIZE(g.IO.MouseDown); i++)
     {
         if (g.IO.MouseClicked[i])
             g.IO.MouseDownOwned[i] = (g.HoveredWindow != NULL);