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
playback: Add subpicture/x-dvb as raw caps

https://bugzilla.gnome.org/show_bug.cgi?id=710325
sdp: fix duplicate 'const' declaration warnings

https://bugzilla.gnome.org/show_bug.cgi?id=711258
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
audio-format: Use ORC for filling memory with silence samples
audio: Update ORC dist files
audio: Add unit test for filling memory with silence samples
discoverer: fix -Wformat-nonliteral warning
discoverer: fix build after last commit

Add a forward declaration for my_g_string_append_printf that specifies
G_GNUC_PRINTF. Turn off indent on it as it drives gst-indent crazy.