fix for ofTrueTypeFont reloading textures only after a successful load.

added bLastFontLoadedOk flag which keeps track of a successful font
load.
when reloadTextures() is called, it only reloads the textures if the
font was previously loaded.
this fix stops this error message on iOS,
[ error ] ofTrueTypeFont: loadFontFace(): couldn't create new face for
"": FT_Error 2 unknown freetype