Text-animation: fix missing encoding specification

Ensure we can pass full UTF-8 strings into text animations. Will consider
for back-port after discussion on the devel list.

Ticket-Id: https://sourceforge.net/p/flightgear/codetickets/2512/

simgear/scene/model/SGText.cxx

@@ -80,7 +80,7 @@ void SGText::UpdateCallback::operator()(osg::Node * node, osg::NodeVisitor *nv )
     // be lazy and set the text only if the property has changed.
     // update() computes the glyph representation which looks 
     // more expensive than a the above string compare.
-    text->setText( buf );
+    text->setText( buf, osgText::String::ENCODING_UTF8 );
     text->update();
   }
   traverse( node, nv );