Export GES library API in headers when we're building the
library itself, otherwise import the API from the headers.
This fixes linker warnings on Windows when building with MSVC.
Fix up some missing config.h includes when building the lib which
is needed to get the export api define from config.h
Fixes https://gitlab.freedesktop.org/gstreamer/gst-editing-services/issues/42
Otherwise we could have not negotiated errors in audiomixer when
the channel/channel-mask do not match
Differential Revision: https://phabricator.freedesktop.org/D493
Reviewed-by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>
Unref pads_info hash table in dispose instead of
finalize, i.e. before gst_bin_dispose runs and
destroys pads_info->bin (to which the pads_info
does not hold a ref).
