Don't reset the segment because we need the values for accumulation. the segment
is reset at start and after a flushing seek. Fixes some problems with files with
quicktime segments.

When we start and renegotiate, there is a moment where the stream is created but
not yet connected. Make sure all functions deal with this situation correctly
instead of erroring out.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=681247

When get_time is called but we are not yet negotiated, return 0 instead of
posting an error. It's possible that the base class is still negotiating when
our get_time is called.

If it ever turns out that we really must use thoe specific
fourccs and not the generic one, we can still add a flavor
field to the caps later.

When setup fails, make sure to cleanup afterwards.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=673509

... thereby plugging a memleak.

... now implemented by buffer memory blocks.

Add bufferpool support to avoid a memcpy in the videosink when actively
interlacing.
Remove some commented obsolete code.

We can let the allocation query pass when we are operating in passthrough mode.

instead of blindly forwarding unknown events.

Call the default query handler instead of forwarding the query blindly. Fixes
issues of strides because of proxying the allocation query wrongly.

_update_properties takes the object lock and should not be called when the
object lock is already taken.

instead of the old boolean "interlaced" field.

Receiving an RTCP packet should also stop probation on sources we have seen
before.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=683065

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=682959

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=681369