drm: notice a new subscriber while the screen is still

Once a still screen has used up its repeats and the static refresh
budget, the WouldBlock arm no longer reaches handle_one_frame or a
static refresh encode, where a subscriber added without a refresh
(the mobile display switch) is noticed. That client got no picture
until the screen changed.

Run the same new-subscriber check at the top of the WouldBlock arm.

src/server/video_service.rs

@@ -900,6 +900,15 @@ fn run(vs: VideoService) -> ResultType<()> {
 
         match res {
             Err(ref e) if e.kind() == WouldBlock => {
+                // A still screen may never reach handle_one_frame, which is where a subscriber
+                // added without a refresh (mobile display switch) is otherwise noticed.
+                sp.snapshot(|sps| {
+                    if sps.has_subscribes() {
+                        log::info!("switch due to new subscriber");
+                        bail!("SWITCH");
+                    }
+                    Ok(())
+                })?;
                 #[cfg(windows)]
                 if dxgi_recovery_started
                     .map(|started| started.elapsed() >= DXGI_RECOVERY_FRAME_GRACE)