Base classes
------------

GstBaseSink
 
  Base class for sink elements.

  - one sinkpad
  - handles state changes
  - does flushing
  - preroll with optional preview
  - pull/push mode
  - EOS handling

  FIXME: not much point making it operate in pull mode as a generic
  base class I guess... 

GstBaseTransform

  Base class for simple tranform filters

  - one sinkpad and one srcpad
  - possible formats on sink and source pad implemented
    with custom transform_caps function. By default uses 
    same format on sink and source.
  - handles state changes
  - does flushing
  - push mode
  - pull mode if transform can operate on arbitrary data

GstBaseSrc

  Base class for getrange based source elements

  - one sinkpad
  - handles state changes
  - pull/push mode
  - handles seeking/query

GstPushSrc

  Base class for push based source elements