ttf: binded TTF_FontFaceStyleName in sdl_ttf.go (#553)

Binded TTF_FontFaceStyleName to FaceStyleName() on the Font struct.
Note: TTF_FontFaceStyleName was not supported until SDL_ttf 2.0.4.

ttf/sdl_ttf.go

@@ -349,6 +349,14 @@ func (f *Font) FaceFamilyName() string {
 	return fname
 }
 
+// FaceStyleName returns the current font face family's style name from the loaded font.
+// (https://wiki.libsdl.org/SDL_ttf/TTF_FontFaceStyleName)
+func (f *Font) FaceStyleName() string {
+        _fname := C.TTF_FontFaceStyleName(f.f)
+        fname := C.GoString(_fname)
+        return fname
+}
+
 // GlyphMetrics contains glyph-specific rendering metrics.
 type GlyphMetrics struct {
 	MinX, MaxX int