Implements a message handling function to preview pipeline bus.
If GST_MESSAGE_ERROR is seen, considers preview pipeline unable
to do its job and posts an error message to application.
Sets pipeline element to NULL so that subsequent calls to post_preview
and set_caps functions just returns without pushing anything to the
disposed preview pipeline. Leaves further actions to the application.
Implements a state indicating flag to preview pipeline,
so that new caps are not set if the pipeline is processing a
preview. The caps are set as pending and applied when the
next preview post is called.
In this case a wait was implemented in the post_preview function,
so that new preview image buffer will wait until the other previews
have been posted to the application and the new caps can be used
safely.
