FT_Face is also not thread, protect getTextAsGlyphs

(cherry picked from commit 7b4abb1f7b)

common/font/outline_font.cpp

@@ -335,6 +335,8 @@ VECTOR2I OUTLINE_FONT::getTextAsGlyphs( BOX2I* aBBox, std::vector<std::unique_pt
                                         TEXT_STYLE_FLAGS aTextStyle ) const
 {
     VECTOR2D glyphSize = aSize;
+    
+    std::lock_guard<std::mutex> guard( m_freeTypeMutex );
     FT_Face  face = m_face;
     double   scaler = faceSize();
 
@@ -562,6 +564,9 @@ void OUTLINE_FONT::RenderToOpenGLCanvas( KIGFX::OPENGL_GAL& aGal, const wxString
     unsigned int         glyphCount;
     hb_glyph_info_t*     glyphInfo = hb_buffer_get_glyph_infos( buf, &glyphCount );
     hb_glyph_position_t* glyphPos = hb_buffer_get_glyph_positions( buf, &glyphCount );
+    
+    std::lock_guard<std::mutex> guard( m_freeTypeMutex );
+
     hb_font_t*           referencedFont = hb_ft_font_create_referenced( m_face );
 
     hb_ft_font_set_funcs( referencedFont );

include/font/outline_font.h

@@ -137,9 +137,14 @@ protected:
 
 private:
     // FreeType variables
+
+    /**
+     * Mutex for freetype access, FT_Library and FT_Face are not thread safe
+     */
     static std::mutex m_freeTypeMutex;
     static FT_Library m_freeType;
     FT_Face           m_face;
+
     const int         m_faceSize;
     bool              m_fakeBold;
     bool              m_fakeItal;