gst: libcamerasrc: Implement initial streaming

With this patch, the element is now able to push buffers to the next
element in the graph. The buffers are currently missing any metadata
like timestamp, sequence number. This will be added in the next commit.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/gstreamer/gstlibcamerapad.cpp

@@ -19,6 +19,7 @@ struct _GstLibcameraPad {
 	StreamRole role;
 	GstLibcameraPool *pool;
 	GQueue pending_buffers;
+	GstClockTime latency;
 };
 
 enum {
@@ -164,3 +165,11 @@ gst_libcamera_pad_push_pending(GstPad *pad)
 
 	return gst_pad_push(pad, buffer);
 }
+
+bool
+gst_libcamera_pad_has_pending(GstPad *pad)
+{
+	auto *self = GST_LIBCAMERA_PAD(pad);
+	GLibLocker lock(GST_OBJECT(self));
+	return self->pending_buffers.length > 0;
+}