Update previews on preference change (#9507)

Normally the theBufferList().updatePreviews(); statement would go inside
of PrefDisplay::applyRC. However, that would not work well because
Buffer::updatePreviews() calls loader() which conditions on
(lyxrc.preview == LyXRC::PREVIEW_OFF) but the new RC has not been set at
this point.

src/BufferList.cpp

@@ -344,6 +344,15 @@ void BufferList::recordCurrentAuthor(Author const & author)
 }
 
 
+void BufferList::updatePreviews()
+{
+	BufferStorage::iterator it = bstore.begin();
+	BufferStorage::iterator end = bstore.end();
+	for (; it != end; ++it)
+		(*it)->updatePreviews();
+}
+
+
 int BufferList::bufferNum(FileName const & fname) const
 {
 	FileNameList const buffers(fileNames());

src/BufferList.h

@@ -115,6 +115,8 @@ public:
 	//@{
 	/// reset current author for all buffers
 	void recordCurrentAuthor(Author const & author);
+	/// update previews for all buffers, e.g. for Prefs update
+	void updatePreviews();
 	/// Call changed() on all buffers, internal or not
 	void changed(bool update_metrics) const;
 	/// emergency save for all buffers

src/frontends/qt4/GuiPrefs.cpp

@@ -3456,6 +3456,8 @@ void GuiPreferences::dispatchParams()
 		update_screen_font_ = false;
 	}
 
+	theBufferList().updatePreviews();
+
 	// The Save button has been pressed
 	if (isClosing())
 		dispatch(FuncRequest(LFUN_PREFERENCES_SAVE));