[wpigui] Limit frame rate to 120 fps by default (#5030)

Limiting with vsync is apparently unreliable on a number of systems;
this resulted in high CPU/GPU usage.

Also add current actual frame rate to about dialog of GUI tools.

wpigui/src/main/native/include/wpigui.h

@@ -162,6 +162,13 @@ enum Style { kStyleClassic = 0, kStyleDark, kStyleLight };
  */
 void SetStyle(Style style);
 
+/**
+ * Sets the FPS limit.  Using this function makes this setting persistent.
+ *
+ * @param fps FPS (0=vsync)
+ */
+void SetFPS(int fps);
+
 /**
  * Sets the clear (background) color.
  *