[client] imgui: render twice for alerts

When using jitRender, or on the first frame of an alert the window
doesn't get resized immediately causing it to cut off the end of the
text.

ImGui needs two passes to calulate the bounding box for automatically
sized windows, this is per it's design and not a bug, see:

https://github.com/ocornut/imgui/issues/2158#issuecomment-434223618

client/src/main.h

@@ -61,6 +61,7 @@ struct AppState
   bool             modAlt;
   bool             modSuper;
   uint64_t         lastImGuiFrame;
+  bool             renderImGuiTwice;
 
   bool        alertShow;
   char      * alertMessage;