cache: mild auto-cache overhaul

Shifted from CacheTask to functionality built into RenderBackend. It was
a lot easier to control behavior this way without having to juggle a ton
of threads and race conditions.

Could likely be multithreaded further.

app/task/cache/cache.cpp

@@ -72,7 +72,7 @@ QFuture<void> CacheTask::DownloadFrame(FramePtr frame, const QByteArray &hash)
 void CacheTask::FrameDownloaded(const QByteArray &hash, const std::list<rational> &times)
 {
   foreach (const rational& t, times) {
-    viewer()->video_frame_cache()->SetHash(t, hash, job_time());
+    viewer()->video_frame_cache()->SetHash(t, hash, job_time(), true);
   }
 }