This allows correct output pix_fmt auto-selection if the input pix_fmt is yuva420. Fixes ticket #290.
@@ -109,6 +109,7 @@ static const PixFmtInfo pix_fmt_info[PIX_FMT_NB] = {
/* YUV formats with alpha plane */
[PIX_FMT_YUVA420P] = {
+ .is_alpha = 1,
.color_type = FF_COLOR_YUV,
},