onRender 
  fun onRender(connectFlags: <Error class: unknown class> = ConnectFlags(0u), handler: (<Error class: unknown class>) -> Boolean): <Error class: unknown class>(source)
Emitted every time the contents of the GtkGLArea should be redrawn.
The @context is bound to the @area prior to emitting this function, and the buffers are painted to the window once the emission terminates.
Parameters
connectFlags 
a combination of ConnectFlags
handler
the Callback to connect. Params: context the GdkGLContext used by @area. Returns true to stop other handlers from being invoked for the event. false to propagate the event further.