[client] renderer: add ability to toggle the FPS display

Before, if you want to see the FPS, you need to close the client and
restart it with the -k switch to see the FPS. This is annoying.

This PR introduces a new keybind, ScrollLock+D, which, when pressed,
toggles the display of the FPS.

This is implemented for both EGL and OpenGL backends.

client/src/main.h

@@ -47,6 +47,7 @@ struct AppState
 
   bool                 stopVideo;
   bool                 ignoreInput;
+  bool                 showFPS;
   bool                 escapeActive;
   int                  escapeAction;
   KeybindHandle        bindings[KEY_MAX];