gstreamer: Implement renegotiation

This commit implements renegotiation of the camera configuration and
source pad caps. A renegotiation can happen when a downstream element
decides to change caps or the pipeline is dynamically changed.

To handle a renegotiation the GST_FLOW_NOT_NEGOTIATED return value has
to be handled in GstLibcameraSrcState::processRequest(). Otherwise the
default would be to print an error and stop streaming.

To archive this in a clean way the if statement is altered into a switch
statement which now also has a case for GST_FLOW_NOT_NEGOTIATED. In the
case of GST_FLOW_NOT_NEGOTIATED every source pad is checked for the
reconfiguration flag with gst_pad_needs_reconfigure() which does not
clear this flag. If at least one pad requested a reconfiguration the
function returns without an error and the renegotiation will happen
later in the running task. If no pad requested a reconfiguration then
the function will return with an error.

In gst_libcamera_src_task_run() the source pads are checked for the
reconfigure flag by calling gst_pad_check_reconfigure() and if one pad
returns true and the caps are not sufficient anymore then the
negotiation is triggered. It is fine to trigger the negotiation after
only a single pad returns true for gst_pad_check_reconfigure() because
the reconfigure flags are cleared in the gst_libcamera_src_negotiate()
function.

If any pad requested a reconfiguration the following will happen:
1. The camera is stopped because changing the configuration may not
   happen while running.
2. The completedRequests queue will be cleared by calling
   GstLibcameraSrcState::clearRequests() because the completed buffers
   have the wrong configuration.
3. The new caps are negotiated by calling gst_libcamera_src_negotiate().
   When the negotiation fails streaming will stop.
4. The camera is started again.

Signed-off-by: Jaslo Ziska <jaslo@ziska.de>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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