[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/include/app.h

@@ -115,4 +115,9 @@ void app_releaseAllKeybinds(void);
  */
 void app_showHelp(bool show);
 
+/**
+ * Changes whether the FPS is displayed or not.
+ */
+void app_showFPS(bool showFPS);
+
 #endif