The returned #GstTaskPool is used internally for performing parallel video format conversions/scaling/etc during the #GstVideoAggregatorPadClass::prepare_frame_start() process. Subclasses can add their own operation to perform using the returned #GstTaskPool during #GstVideoAggregatorClass::aggregate_frames().
#GstGLBaseMixer handles the nitty gritty details of retrieving an OpenGL context. It provides some virtual methods to know when the OpenGL context is available and is not available within this element.