Expose aliasing method as an option

It's an RGB texture either way, which is why you can just toggle it
easily.

src/OpenGLFontAtlas.cake

@@ -180,9 +180,8 @@ void main()
      font-data (addr (const (unsigned char))) font-data-size (unsigned int)
      device-dpi (unsigned int)
      font-size-points (unsigned char)
+     enable-subpixel-antialiasing bool
      &return bool)
-  ;; TODO: This won't make sense for any text rendered in 3D
-  (var enable-subpixel-antialiasing bool true)
   (unless (= 0 (build-font-atlas font-data font-data-size
                                  device-dpi
                                  font-size-points