Add wrapper for GstBaseTransform
See https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer-libs/html/GstBaseTransform.html Might make sense to do this first: https://github.com/sdroege/gstreamer-rs/issues/7
First step would be to come up with an API design (mostly the main trait) in the style of
Demuxer. That is:
- all functions return a value and don't call into other elements. So return value would be some kind of enum
- the general idea of the C base class would be kept (i.e. you have some kind of
- implementors of the trait do not have to worry about threading
As part of this should also be a simple transform element. Maybe just a minimal "volume" element, or mono to stereo converter (the latter would at least make use of caps transformations).