FcFontRenderPrepare - Prepare pattern for loading font file
#include <fontconfig/fontconfig.h>
FcPattern * FcFontRenderPrepare (FcConfig *config, FcPattern *pat, FcPattern *font);
Creates a new pattern consisting of elements of font not appearing in
  pat, elements of pat not appearing in font and the best
  matching value from pat for elements appearing in both. The result is
  passed to FcConfigSubstituteWithPat with kind FcMatchFont and then
  returned.