Update functions to return SamplePtr instead of BufferPtr.

gst_app_sink_pull_preroll and gst_app_sink_pull_sample return a SamplePtr
gst_app_sink_pull_buffer renamed to gst_app_sink_pull_sample
gst_app_sink_pull_buffer_list removed
GstVideoInfo provides most of BufferFormats previous members.

I moved everything to be wrappers around GstVideoInfo though
mostly left the API alone.