Fix vari-speed and non-locked slave modes

A transport-speed-change is no reason to skip processing.

Prior to this change cannot_process() silently skipped in the process
cycle of the speed-change (which may have been every process-cycle),
without moving the transport or doing any processing.

libs/ardour/ardour/session.h

@@ -1443,7 +1443,6 @@ class LIBARDOUR_API Session : public PBD::StatefulDestructible, public PBD::Scop
 	static const PostTransportWork ProcessCannotProceedMask =
 		PostTransportWork (
 			PostTransportInputChange|
-			PostTransportSpeed|
 			PostTransportReverse|
 			PostTransportCurveRealloc|
 			PostTransportAudition|

libs/ardour/session_process.cc

@@ -674,7 +674,7 @@ Session::follow_slave (pframes_t nframes)
 	}
 
 
-	if (_slave_state == Running && !non_realtime_work_pending()) {
+	if (_slave_state == Running && 0 == (post_transport_work () & ~PostTransportSpeed)) {
 		/* speed is set, we're locked, and good to go */
 		return true;
 	}