[client] imgui: fix modifier key modification race

imgui really hates it when we update the modifier key state after igNewFrame.
The result is:

    void ImGui::ErrorCheckEndFrameSanityChecks(): Assertion
    `(key_mod_flags == 0 || g.IO.KeyMods == key_mod_flags) &&
    "Mismatching io.KeyCtrl/io.KeyShift/io.KeyAlt/io.KeySuper vs io.KeyMods"'
    failed.

Therefore, we buffer the modifier state information and update it in the IO
object right before we call igNewFrame.

client/src/main.h

@@ -56,6 +56,10 @@ struct AppState
   bool             overlayInput;
   ImGuiMouseCursor cursorLast;
   char           * imGuiIni;
+  bool             modCtrl;
+  bool             modShift;
+  bool             modAlt;
+  bool             modSuper;
 
   bool        alertShow;
   char      * alertMessage;