use lossless compression on indexed frames

We use Qt's built in zlib compression on the fastest setting (it was found that
higher compression took 5x as long with a negligible decrease in size). This
helps keep disk cache sizes low.

app/render/diskmanager.h

@@ -1,6 +1,7 @@
 #ifndef DISKMANAGER_H
 #define DISKMANAGER_H
 
+#include <QMutex>
 #include <QObject>
 
 class DiskManager : public QObject
@@ -31,7 +32,7 @@ private:
 
   static DiskManager* instance_;
 
-  void DeleteLeastRecent();
+  QByteArray DeleteLeastRecent();
 
   qint64 DiskLimit();
 
@@ -46,6 +47,8 @@ private:
 
   qint64 consumption_;
 
+  QMutex lock_;
+
 };
 
 #endif // DISKMANAGER_H