PNG_COLOR_MASK_COLOR

The commented out code is obsolete since commit https://github.com/pornel/pngquant/commit/4f29636e4737989707e4c46a14375d4fe092bebf anyway. This is also to have consistency with another forthcoming addition to the lcms code.

```png.h```
```C
/* color types.  Note that not all combinations are legal */
#define PNG_COLOR_TYPE_GRAY 0
#define PNG_COLOR_TYPE_PALETTE  (PNG_COLOR_MASK_COLOR | PNG_COLOR_MASK_PALETTE)
#define PNG_COLOR_TYPE_RGB        (PNG_COLOR_MASK_COLOR)
#define PNG_COLOR_TYPE_RGB_ALPHA  (PNG_COLOR_MASK_COLOR | PNG_COLOR_MASK_ALPHA)
#define PNG_COLOR_TYPE_GRAY_ALPHA (PNG_COLOR_MASK_ALPHA)
```