We cannot create a ContextReader in the filter background thread

This is because ContextReader gets the activeSite, which (in case of
the UIContext) asks to the UI thread this information (app::Editor).

src/app/commands/filters/filter_manager_impl.cpp

@@ -50,8 +50,8 @@ using namespace std;
 using namespace ui;
 
 FilterManagerImpl::FilterManagerImpl(Context* context, Filter* filter)
-  : m_context(context)
-  , m_site(context->activeSite())
+  : m_reader(context)
+  , m_site(*const_cast<Site*>(m_reader.site()))
   , m_filter(filter)
   , m_cel(nullptr)
   , m_src(nullptr)
@@ -281,8 +281,7 @@ void FilterManagerImpl::applyToTarget()
   }
 
   // Initialize writting operation
-  ContextReader reader(m_context);
-  ContextWriter writer(reader);
+  ContextWriter writer(m_reader);
   m_tx.reset(new Tx(writer.context(), m_filter->getName(), ModifyDocument));
 
   m_progressBase = 0.0f;

src/app/commands/filters/filter_manager_impl.h

@@ -10,6 +10,7 @@
 #pragma once
 
 #include "app/commands/filters/cels_target.h"
+#include "app/context_access.h"
 #include "app/site.h"
 #include "app/tx.h"
 #include "base/exception.h"
@@ -139,8 +140,8 @@ namespace app {
     void redrawColorPalette();
 #endif
 
-    Context* m_context;
-    Site m_site;
+    ContextReader m_reader;
+    Site& m_site;
     Filter* m_filter;
     doc::Cel* m_cel;
     doc::ImageRef m_src;

src/app/commands/filters/filter_worker.cpp

@@ -12,6 +12,7 @@
 #include "app/app.h"
 #include "app/commands/filters/filter_manager_impl.h"
 #include "app/console.h"
+#include "app/context_access.h"
 #include "app/i18n/strings.h"
 #include "app/ini_file.h"
 #include "app/modules/editors.h"