Revert "decodebin2: use NO_RESYNC flag"

This reverts commit 0feecef2754ef208372eb39332b4f6fa2067d3d5.
decodebin: Lock the state of child elements as long as we manage their states

https://bugzilla.gnome.org/show_bug.cgi?id=690420
video: Always provide a buffer in gst_video_(enc|dec)oder_allocate_output_buffer()

We have no way of tell the caller of the exact error (e.g. if we're flushing),
so will have to wait until the caller uses API that returns a GstFlowReturn,
for example when pushing this buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=700006
audio: Always provide a buffer in gst_audio_(enc|dec)oder_allocate_output_buffer()

We have no way of tell the caller of the exact error (e.g. if we're flushing),
so will have to wait until the caller uses API that returns a GstFlowReturn,
for example when pushing this buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=700006
typefind: Ensure we have enough data when reading the sync marker in the AAC/LOAS typefinder
Start on adding stream combiners to playbin.
Fix *-stream-combiner properties to be objects.
Use custom stream combiners when setting up groups.
Only set always-ok on pads that have that property.
Don't remove custom stream combiners when there are no pads left.
Don't allow setting the current stream if the stream combiner isn't an input-selector.
Don't automatically add and remove stream combiners to the playbin bin.
Setup custom stream combiners in pad_added_cb instead of activate_group.
Check and cache stream combiner and sink pad properties up-front, and don't attempt anything that
requires those properties (current-*, tags, etc.).