chan_sip: Only send video on outgoing channel if incoming channel supports it

Previously, the settings videosupport=always and videosupport=yes behaved
identically and unconditionally caused a video offer to be sent in the SDP on
an outgoing call. This was a regression introduced with commit
5a1d90e1 in Asterisk 1.6.1.

This commit restores correct behavior: videosupport=always causes a video offer
to be sent unconditionally, while videosupport=yes will only offer video on an
outbound channel if the incoming channel it is bridged to also supports video.
That way, the device receiving the outgoing call can display the correct user
interface elements for audio or video and will not unnecessarily show a blank
video window on an audio-only call.

ASTERISK-17470 #close

Change-Id: I782f4409d436114dbc97061c3570c0cd24f7c3ae