`avifdec in.avif out.png` adds alpha channel even when the input doesn’t have one

[da2x]

avifdec any-image.avif output.png always adds an alpha channel in the output PNG regardless of whether it’s needed or not.

Version: 0.9.2 (dav1d [dec]:0.9.1, aom [enc/dec]:v3.1.2, rav1e [enc]:0.4.1 (UNKNOWN), svt [enc]:0.8.6)
libyuv : unavailable

[wantehchang]

@FrankGalligan I wrote a fix at #1021.

@joedrago I found that PNG also has PNG_COLOR_TYPE_GRAY and PNG_COLOR_TYPE_GRAY_ALPHA, so AVIF monochrome (YUV 4:0:0) images could be encoded in those two PNG color types.