add font render type to choose

add font type, users can choose FTGLBitmap, FTGLBuffer, FTGLExtrd, FTGLOutline, FTGLPixmap FTGLPolygon and FTGLTexture to render text. When I create a font with size 100, and use default FTGLTexture, ftgl crushs in FTTextureFontImpl::CreateTexture because new unsigned char[totalMemory] throw an exception. But use FTGLPolygon done well.