Avoid possible race condition on NIFFile::sLoadUnsupportedFiles

Its value is written from the main thread but other threads read it.

components/nif/niffile.cpp

@@ -344,7 +344,7 @@ bool NIFFile::getUseSkinning() const
     return mUseSkinning;
 }
 
-bool NIFFile::sLoadUnsupportedFiles = false;
+std::atomic_bool NIFFile::sLoadUnsupportedFiles = false;
 
 void NIFFile::setLoadUnsupportedFiles(bool load)
 {

components/nif/niffile.hpp

@@ -5,6 +5,7 @@
 
 #include <stdexcept>
 #include <vector>
+#include <atomic>
 
 #include <components/debug/debuglog.hpp>
 #include <components/files/constrainedfilestream.hpp>
@@ -65,7 +66,7 @@ class NIFFile final : public File
 
     bool mUseSkinning = false;
 
-    static bool sLoadUnsupportedFiles;
+    static std::atomic_bool sLoadUnsupportedFiles;
 
     /// Parse the file
     void parse(Files::IStreamPtr stream);