videoscale: fix adding borders when NV12 is used

When the frame buffer is NV12 the borders are not added at all, fix that
and fill them to black.

https://bugzilla.gnome.org/show_bug.cgi?id=711003
rtspconnection: allow setting tls certificate validation

Added new functions gst_rtsp_connection_set_tls_validation_flags() to
allow setting the TLS certificate validation flags when establishing a
TLS connection.
A getter is also available, gst_rtsp_connection_get_tls_validation_flags().

https://bugzilla.gnome.org/show_bug.cgi?id=711231
rtspconnection: Fix indention in header
rtspconnection: Add new API to the docs and .def file