Screensharing signalling feature.

    A client can ebale screensharing by setting the isScreensharing
    flag in the call parameters to TRUE. This change will only take into
    effect if the call and the conference support video capabilities.
    As far as the signalling is concerned, the SDK will add or modify the
    content of the main video stream to "slides" if the call is in a
    conference.

    Screensharing can be enabled for any layout the client is in.

    In case one or more clients wish to share the screen, the last one
    making its request will actually share the screen.

    The application has the responsability to set the video source
    descriptor corresponding to the screen, window or program to share. This
    can be done eithe rbefore or after updating the call parameters.
    Similarly, once the video scharing session ends, the application must
    change the video source descriptor.

    The person who shared its screen can avoid sharing the image of its
    camera by diabling the camera in the call parameters

    The application will be notified if the screensharing request was
    accepted through a dedicated callback

liblinphone @ ccb753a0

-Subproject commit 9ce4d29e6554934d39754fd25d1244713cffa0ad
+Subproject commit ccb753a0797c9e18af98ff5c528e7a8a94011fd9

mediastreamer2 @ 861a9a46

-Subproject commit 3c2965d001e9f497431041595e5ad1b65c444cb6
+Subproject commit 861a9a46342de4f525cc137f0c0b92cac719ee76