video: disconnect preview

lambda registered for frameUpdated was still alive after closing the window.
It was consuming frames, preventing the call to get them.
It's a limitation of our implementetation: there is only one possible receiver
of frameUpdated for a given renderer.

Tuleap: #130
Change-Id: Ic8f5b8efea60e3409c77e4510320e3eba11fe21d

src/VideoPrefsVC.mm

@@ -99,17 +99,12 @@ QMetaObject::Connection previewStopped;
 
     }
 
-    // check if preview has to be started/stopped by this controller
-    self.shouldHandlePreview = !Video::PreviewManager::instance().isPreviewing();
-
     [previewView setWantsLayer:YES];
     [previewView setLayer:[CALayer layer]];
     [previewView.layer setBackgroundColor:[NSColor blackColor].CGColor];
     [previewView.layer setContentsGravity:kCAGravityResizeAspect];
     [previewView.layer setFrame:previewView.frame];
     [previewView.layer setBounds:previewView.frame];
-
-    [self connectPreviewSignals];
 }
 
 - (IBAction)chooseDevice:(id)sender {
@@ -135,6 +130,7 @@ QMetaObject::Connection previewStopped;
     QObject::disconnect(frameUpdated);
     QObject::disconnect(previewStopped);
     QObject::disconnect(previewStarted);
+
     previewStarted = QObject::connect(&Video::PreviewManager::instance(),
                                       &Video::PreviewManager::previewStarted,
                                       [=](Video::Renderer* renderer) {
@@ -197,6 +193,10 @@ QMetaObject::Connection previewStopped;
 
 - (void) viewWillAppear
 {
+    // check if preview has to be started/stopped by this controller
+    self.shouldHandlePreview = !Video::PreviewManager::instance().previewRenderer()->isRendering();
+
+    [self connectPreviewSignals];
     if (self.shouldHandlePreview) {
         Video::PreviewManager::instance().startPreview();
     }
@@ -204,6 +204,9 @@ QMetaObject::Connection previewStopped;
 
 - (void)viewWillDisappear
 {
+    QObject::disconnect(frameUpdated);
+    QObject::disconnect(previewStopped);
+    QObject::disconnect(previewStarted);
     if (self.shouldHandlePreview) {
         Video::PreviewManager::instance().stopPreview();
     }