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This is like VideoFrame, but can work on gst::BufferRefs and borrows
from it. VideoFrames can be converted into this by borrowing.

Not very useful, but will be useful for gst-plugin-rs

Values are passed owned to the callback

For consistency with the C API naming.

Should fix https://github.com/sdroege/gstreamer-rs/issues/68

Like all GObjects, mutation works through interior mutability.

This can be used to propagate an element error via a Result until the
place where it can be actually posted on an element.

It means that there were no changes.

Fixes https://github.com/sdroege/gstreamer-rs/issues/11

It's immutable.

It's useless and not used.