Building against mfx_dispatcher is used to search for
headers in PREFIX/include/mfx/ only (commit: 62f04e801bd7e247102ac67df889bee33ab74ff7),
but it is just PREFIX/include with open source msdk version.
https://bugzilla.gnome.org/show_bug.cgi?id=796118
The base encoder common properties are not valid for
mjpeg encoder where there is no motion compensation or rate control.
Delaying the property installation on the base gobject
untill the subclass class_init get invoked.
https://bugzilla.gnome.org/show_bug.cgi?id=791637
Makes GstMsdkContext to be a descendant of GstObject so that
we could track the life-cycle of the session of the driver.
Also replaces MsdkContext with this one.
Keeps msdk_d3d.c alive for the future.
https://bugzilla.gnome.org/show_bug.cgi?id=790752