Subclass: parent class optional functions invocation

This is an attempt at defining a behaviour for the invocation of the parent's function from a subclass. The discussion started here: !216 (comment 108728).

I proceeded like this:

• List all the parent_xxx functions under gstreamer and gstreamer-base.
• Search in the xxx_class_init C functions whether a default function was assigned to a vfunc.
• Search the documentation whether the vfunc is mandatory (which is actually kind of useless for this analysis, but I did the search, so I included the result).
• I also listed a couple of default base functions which I think an implementer can't call from the Rust binding.

I don't get why we have functions such as Aggregator.parent_start. AIUI, in an Aggregator, there is no start default function. When it's defined, the start vfunc is called by gst_aggregator_start, which is called by gst_aggregator_change_state which is bound to the vfunc state_change of the parent (GstElementClass).

ATM, my understanding is that we should:

• Remove parent_xxx when xxx doesn't have a default. In other words, if xxx_class_init didn't initialize a function in the base class, I don't think we can expect to find one when invoking:

let parent_class = data.as_ref().get_parent_class() as *mut ffi::GstAggregatorClass;
(*parent_class)
    .start
    [...]

• Have parent_xxx functions which have a default expect() the function to exist.

gstreamer

Function	Mandatory	Has Default
Bin.parent_add_element		x
Bin.parent_handle_message		x
Bin.parent_remove_element		x
Element.parent_change_state		x
Element.parent_query		x
Element.parent_send_event		x
Element.parent_set_context		x
Pad.parent_linked
Pad.parent_unlinked

Not Defined

The base C classe define these default functions and assign them to the vfunc, but AFAICT implementers have no way of calling the parent (default).

Function
Bin.deep_element_added
Bin.deep_element_removed
Bin.do_latency

gstreamer-base

Function	Mandatory	Has Default
Aggregator.parent_aggregate	x
Aggregator.parent_clip
Aggregator.parent_create_new_pad		x
Aggregator.parent_finish_buffer		x
Aggregator.parent_fixate_src_caps		x
Aggregator.parent_flush
Aggregator.parent_get_next_time
Aggregator.parent_negotiated_src_caps		x
Aggregator.parent_sink_event		x
Aggregator.parent_sink_query		x
Aggregator.parent_src_activate
Aggregator.parent_src_event		x
Aggregator.parent_src_query		x
Aggregator.parent_start
Aggregator.parent_stop
Aggregator.parent_update_src_caps		x
AggregatorPad.parent_flush
AggregatorPad.parent_skip_buffer
BaseSink.parent_event		x
BaseSink.parent_fixate		x
BaseSink.parent_get_caps		x
BaseSink.parent_query		x
BaseSink.parent_set_caps		x
BaseSrc.parent_create	x	x
BaseSrc.parent_do_seek		x
BaseSrc.parent_event		x
BaseSrc.parent_fixate		x
BaseSrc.parent_get_caps		x
BaseSrc.parent_negotiate		x
BaseSrc.parent_query		x
BaseSrc.parent_set_caps
BaseTransform.parent_accept_caps		x
BaseTransform.parent_fixate_caps		x
BaseTransform.parent_query		x
BaseTransform.parent_sink_event		x
BaseTransform.parent_src_event		x
BaseTransform.parent_transform_caps		x
BaseTransform.parent_transform_size		x

Not Defined

The base C classes define these default functions and assign them to the vfunc, but no direct call nor parent_ functions are defined in the Rust binding.

Function
BaseSink.activate_pull
BaseSink.get_times
BaseSink.wait_event
BaseSrc.alloc
BaseSrc.decide_allocation
BaseSrc.prepare_seek_segment
BaseTransform.copy_metadata
BaseTransform.decide_allocation
BaseTransform.generate_output
BaseTransform.propose_allocation
BaseTransform.prepare_output_buffer
BaseTransform.submit_input_buffer
BaseTransform.transform_meta