openh264: add slice support

This MR adds the support of openH264 slicing mode.

$ gst-launch-1.0 -v videotestsrc num-buffers=10 ! openh264enc num-slices=8 ! openh264dec ! autovideosink

ext/openh264/gstopenh264dec.cpp

@@ -61,9 +61,9 @@ static gboolean gst_openh264dec_decide_allocation (GstVideoDecoder * decoder,
     GstQuery * query);
 
 #if HAVE_OPENH264_MAIN_PROFILE
-#define SUPPORTED_PROFILE_STR "profile=(string){ constrained-baseline, baseline, main, high }"
+#define SUPPORTED_PROFILE_STR ", profile=(string){ constrained-baseline, baseline, main, high }"
 #else
-#define SUPPORTED_PROFILE_STR "profile=(string){ constrained-baseline, baseline }"
+#define SUPPORTED_PROFILE_STR ", profile=(string){ constrained-baseline, baseline }"
 #endif
 
 /* pad templates */
@@ -72,7 +72,7 @@ GST_STATIC_PAD_TEMPLATE ("sink",
     GST_PAD_SINK,
     GST_PAD_ALWAYS,
     GST_STATIC_CAPS
-    ("video/x-h264, stream-format=(string)byte-stream, alignment=(string)au, "
+    ("video/x-h264, stream-format=(string)byte-stream, alignment=(string){ au, nal }"
       SUPPORTED_PROFILE_STR
     ));