avmodel: quit all renderers in the avmodel dtor

- this will prevent a crash on quit while the preview renderer is
  still running

Change-Id: I6f0392d0bb7015339bca3f2d3f6261c93efc8bb0

src/avmodel.cpp

@@ -143,6 +143,10 @@ AVModel::AVModel(const CallbacksHandler& callbacksHandler)
 
 AVModel::~AVModel()
 {
+    std::lock_guard<std::mutex> lk(pimpl_->renderers_mtx_);
+    for (auto r = pimpl_->renderers_.cbegin(); r != pimpl_->renderers_.cend(); ++r) {
+        (*r).second->quit();
+    }
 }
 
 bool