rendertask: always wait for watchers to finish

I don't know why this wasn't written to do this before, perhaps I thought waiting for the thread would achieve this goal. Anyway, in my testing, this ensures render jobs are complete before the task returns.

Fixes #1899
Fixes #1900

app/task/render/render.cpp

@@ -266,9 +266,14 @@ bool RenderTask::Render(ColorManager* manager,
   if (IsCancelled() || !result) {
     // Cancel every watcher we created
     foreach (RenderTicketWatcher* watcher, running_watchers_) {
+      watcher->Cancel();
       disconnect(watcher, &RenderTicketWatcher::Finished, this, &RenderTask::TicketDone);
       RenderManager::instance()->RemoveTicket(watcher->GetTicket());
     }
+
+    foreach (RenderTicketWatcher* watcher, running_watchers_) {
+      watcher->WaitForFinished();
+    }
   }
 
   watcher_thread.quit();