Closes #12

Part of #12

In autogenerated code these were already replaced but some manual code
still kept them.

Fixes #204

Otherwise it's possible to create a new owned reference from a mutable
reference, and then there is a mutable and immutable reference to the
same data at the same time, which is simply not allowed.

#204

The VideoCodecFrame and VideoCodecState is C API is unfortunatelly unsafe
by design. So we workarounded it by ensuring the decoder stream lock was
hold while user has a writable reference on those objects.

Based on previous work from Thibault Saunier and Philippe Normand.

Fixes #161

Fixes #203

Latest versions of nightly rust compiler can detect unused extern
crates, and this commit removes those marked as unused.

If the probe returns Handled and the data was a Buffer we need to ensure
it was consumed. Queries need to be returned. The behavior of Handled
for other probes is not clear.

This is present since GStreamer 1.5.90 so no feature flags needed.

GStreamer does not allow the probe to consume the reference here.

Fix #201

This allows getting mutable references to the medias so that they can be
modified, which is allowed by the C API and safe to do.

Closes !255

Implemented for element, object, pad, and pipeline

#104

Implemented for Object, Pipeline, Bin, Element, Plugin.

Fixes #104

Fixes #168

I'll use them to bind
gst_video_decoder_allocate_output_frame_with_params()

glib has a futures feature that only works on nighty
and there is no easy to exclude a feature without specifying
every other feature.

Also clean it up a bit.

examples/glupload: Fix segmentation fault by ensuring the glutin context stays alive longer than the GStreamer GL display

Otherwise the GL display might still use a Wayland display that is
already freed.

Fixes #196

Ron changed the way it outputs empty arrays with version `0.5`.