avcodec/png: set correct alpha mode

PNG always uses straight alpha.

cf. https://www.w3.org/TR/PNG-Rationale.html

> Although each form of alpha storage has its advantages, we did not want to
> require all PNG viewers to handle both forms. We standardized on non-
> premultiplied alpha as being the lossless and more general case.

libavcodec/pngdec.c

@@ -998,6 +998,11 @@ static int decode_idat_chunk(AVCodecContext *avctx, PNGDecContext *s,
             s->bpp += byte_depth;
         }
 
+        /* PNG spec mandates independent alpha channel */
+        if (s->color_type == PNG_COLOR_TYPE_RGB_ALPHA ||
+            s->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
+            avctx->alpha_mode = AVALPHA_MODE_STRAIGHT;
+
         ff_progress_frame_unref(&s->picture);
         if (s->dispose_op == APNG_DISPOSE_OP_PREVIOUS) {
             /* We only need a buffer for the current picture. */

libavcodec/pngenc.c

@@ -1255,6 +1255,9 @@ const FFCodec ff_png_encoder = {
                   AV_PIX_FMT_GRAY8, AV_PIX_FMT_GRAY8A,
                   AV_PIX_FMT_GRAY16BE, AV_PIX_FMT_YA16BE,
                   AV_PIX_FMT_MONOBLACK),
+    .alpha_modes    = (const enum AVAlphaMode[]) {
+        AVALPHA_MODE_STRAIGHT, AVALPHA_MODE_UNSPECIFIED
+    },
     .p.priv_class   = &pngenc_class,
     .caps_internal  = FF_CODEC_CAP_ICC_PROFILES,
 };
@@ -1275,6 +1278,9 @@ const FFCodec ff_apng_encoder = {
                   AV_PIX_FMT_PAL8,
                   AV_PIX_FMT_GRAY8, AV_PIX_FMT_GRAY8A,
                   AV_PIX_FMT_GRAY16BE, AV_PIX_FMT_YA16BE),
+    .alpha_modes    = (const enum AVAlphaMode[]) {
+        AVALPHA_MODE_STRAIGHT, AVALPHA_MODE_UNSPECIFIED
+    },
     .p.priv_class   = &pngenc_class,
     .caps_internal  = FF_CODEC_CAP_ICC_PROFILES,
 };