remove const correctness violations

This is possibly related to a livelock where several threads do const
removal in their respective code paths.

Use the `mutable' specifier for the mutexes and spline's cached data.

Now using the `mutable' specifier, get rid of <optional> in
compat/mutex.

1 files changed, 1 insertions, 1 deletions

diff --git a/logic/pipeline.hpp b/logic/pipeline.hpp
index 9fdd6c32..a4dcb6b8 100644
--- a/logic/pipeline.hpp
+++ b/logic/pipeline.hpp
@@ -81,7 +81,7 @@ class OTR_LOGIC_EXPORT pipeline : private QThread
 {
     Q_OBJECT
 
-    QMutex mtx;
+    mutable QMutex mtx;
     main_settings s;
     Mappings& m;
     event_handler& ev;