[pitivi] previewers: Make sure VideoPreviewer is controlled when there is work to do

[Alexandru Băluț <alexbalut src gnome org>]

commit 85de975b6cb6173008dbaad310726f83f11eeed9
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Sun Dec 17 00:14:33 2017 +0100

    previewers: Make sure VideoPreviewer is controlled when there is work to do
    
    The PreviewGeneratorManager class starts the previewers, but once they
    are done, there was nothing to reschedule them for running if the zoom
    changes, etc.
    
    Fixes https://phabricator.freedesktop.org/T7733
    
    Reviewed-by: Thibault Saunier <tsaunier gnome org>
    Differential Revision: https://phabricator.freedesktop.org/D1919

 pitivi/timeline/previewers.py |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index fefee54..1cf6b67 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -295,7 +295,6 @@ Gst.Element.register(None, "teedthumbnailbin", Gst.Rank.NONE,
                      TeedThumbnailBin)
 
 
-# pylint: disable=too-few-public-methods
 class PreviewGeneratorManager(Loggable):
     """Manager for running the previewers."""
 
@@ -612,6 +611,8 @@ class VideoPreviewer(Previewer, Zoomable, Loggable):
             self.remove(thumb)
         self.thumbs = thumbs
         self.queue = queue
+        if queue:
+            self.become_controlled()
 
     def _set_pixbuf(self, position, pixbuf):
         """Sets the pixbuf for the thumbnail at the specified position."""