Delay the creation of video capture until it is needed

[ming]

This can be useful in situations where video camera access is exclusive, so when we don't need it, the video camera can still be used by the user or other apps.

This ticket will include:

• Avoid creating the video capture if the setting vid_out_auto_transmit is false
• Decrease the video capture reference count when the outgoing transmission is disabled (via the API pjsua_call_set_vid_strm() with op PJSUA_CALL_VID_STRM_STOP_TRANSMIT). When the reference count reaches zero (i.e. no other calls nor the preview window is using it), then the video capture will be released.

[ming]

In 5961:

Fixed #2187: Avoid starting video capture if the setting vid_out_auto_transmit is false

[ming]

In 5962:

Re #2187: Delay the creation of video capture until it is needed

Also in this commit: add PJSUA_LOCK() and UNLOCK() around access of pjsua_vid_win