hlsdemux: Directly convert GErrors to error messages

This will make sure that we don't leak debug information into the actual
error message string and keep it behind the debug string.
hlsdemux: Propagate error messages from the source element up in the hierarchy

Instead of inventing our own generic error strings which are mostly useless.
hlsdemux: Include a more descriptive error message
hlsdemux: Include the debug string in the error messages we propagate from the
source
uridownloader: Include the debug string in the error messages we propagate from t source
androidmedia: Clean up flushing code and don't consider output buffer releasing failures during flushing as an error
androidmedia: Don't consider input buffer queueing failures as an error during flushing
androidmedia: Also don't report warnings for queue/dequeue errors during flushing in error cases

... and fix a case where we released an invalid buffer index.
androidmedia: Clear exceptions also when we did not provide a GError
aggregator: Add new GstAggregator base class

This base class has been added to a newly created libgstbadbase library

Co-Authored by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>

https://bugzilla.gnome.org/show_bug.cgi?id=731917
videoaggregator: Create a new GstVideoAggregator baseclass

This base class has been added to a newly created libgstbadvideo library

Co-Authored by: Thibault Saunier <tsaunier@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=731918
gl: Port glmixer to the GstVideoAggregator baseclass

https://bugzilla.gnome.org/show_bug.cgi?id=731921
compositor: Add a new compositor based on the new GstVideoAggregator base class

It is a replacement for videomixer with a similare API

Co-Authored by: Thibault Saunier <tsaunier@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=731919
gl:glvideomixer: Add the Compositor in the element metadata class

So it is possible to pick one compositing element from the registry
libs:video:Fix compilation
build: remove gst-libs/gst/video/ from CRUFT_DIRS

It was gone, but now it's back.
Final fix compilation
libs: videoaggregato: Do not import videoconvert.h in gstvideoaggregatorpad.h

+ Add a Private structure to the GstVideoAggregatorPad
+ Add some padding
aggregator: Fix requested pad name
libs:base: Fix includedir