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The default implementation negotiates a buffer pool and allocator
with downstream.

And also request 16-byte aligned buffers if the subclass didn't
set anything else.

Unifies the code and ensures that:
* subclasses needing to use the frame_number on a void* field will
  always work
* wraparounds will be automatically taken care of if we have to deal
  with more than 2**32 frames

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=680614

This is to be called by encoders once they have set the output format
in order for (re)negotiation to be triggered as early as possible.

https://bugzilla.gnome.org/show_bug.cgi?id=679878

And also renegotiate if the srcpad requires reconfiguration

videoencoder: Add decide_allocation vfunc and functions to allocate buffers with the information from the allocation query

And also the implications of calling the finish() functions.

The frames are the owners of the buffers

https://bugzilla.gnome.org/show_bug.cgi?id=676442

Makes gtk-doc happy.

We have one reference owned by the internal frame list and one reference
passed to the subclass.

This must be handled by the subclasses in 0.11 because interlacing
is much more complex now and can't be handled in a generic way.