h264parse: Fix multiple SEI messages in one SEI RBSP parsing.

An SEI RBSP could contains more than one SEI message as specified in
7.4.2.3.1.

This commit change the parser API: the gst_h264_parser_parse_sei()
function now create and fill a GArray containing GstH264SEIMessage.

https://bugzilla.gnome.org/show_bug.cgi?id=721715
h264parse: Fix multiple SEI messages in one SEI RBSP parsing 2.

Signed-off-by: Zhong Cong <congx.zhong@intel.com>