videotestsrc: Always set pixel-aspect-ratio and interlace-mode in the fixed caps

Otherwise our caps will not be compatible with elements that require a
1/1 pixel-aspect-ratio or progressive video.

https://bugzilla.gnome.org/show_bug.cgi?id=721103
audiobasesrc: Avoid unnecessary configuration

Port a change from audiobasesink from def07410, to ignore setcaps
when the caps don't actually change, and avoid a reconfiguration
and reset of the ringbuffer in that case.
videodec/enc: allow to call negiotate without ouput state

Some decoders call set_output_state from GstVideoDecoder::negotiate()
So move the g_return_val_if_fail to default_negotiate(), i.e. where
it is actually necessary.

Fix https://bugzilla.gnome.org/show_bug.cgi?id=721078