[client] imgui: ensure the pointer shape is correct

Since we only update imgui's cursor location when the overlay is
enabled, if the last cursor position was showing a shape that is
incorrect when we re-enter the overlay the cursor will be wrong. This
corrects this by updating the location as we enter overlay mode.

client/src/app.c

@@ -868,6 +868,8 @@ void app_setOverlay(bool enable)
     wasGrabbed = g_cursor.grab;
 
     g_state.io->ConfigFlags &= ~ImGuiConfigFlags_NoMouse;
+    g_state.io->MousePos = (ImVec2) { g_cursor.pos.x, g_cursor.pos.y };
+
     core_setGrabQuiet(false);
     core_setCursorInView(false);
   }