pngdec: Add support for PIX_FMT_Y400A

Originally committed as revision 22882 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/pngdec.c

@@ -489,6 +489,8 @@ static int decode_frame(AVCodecContext *avctx,
                     avctx->pix_fmt = PIX_FMT_MONOBLACK;
                 } else if (s->color_type == PNG_COLOR_TYPE_PALETTE) {
                     avctx->pix_fmt = PIX_FMT_PAL8;
+                } else if (s->color_type == PNG_COLOR_TYPE_GRAY_ALPHA) {
+                    avctx->pix_fmt = PIX_FMT_Y400A;
                 } else {
                     goto fail;
                 }