Changed Font's mutex to become a static mutex shared between all instances of Font.

This ensures single threaded access of the freetype plugin.

src/osgText/Text.cpp

@@ -464,7 +464,7 @@ void Text::computeGlyphRepresentation()
         return;
     }
     
-    OpenThreads::ScopedLock<Font::FontMutex> lock(*(activefont->getSerializeFontCallsMutex()));
+    OpenThreads::ScopedLock<Font::FontMutex> lock(*(Font::getSerializeFontCallsMutex()));
 
     // initialize bounding box, it will be expanded during glyph position calculation
     _textBB.init();