avcodec/pngdec: do not blend on transparent black

There is no need to memset the zlib output buffer, as there is no
blending happening there. Instead, do not blend when the dispose
operation is set to 'background' (tranparent black).
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngdec.c

@@ -674,10 +674,6 @@ static int decode_idat_chunk(AVCodecContext *avctx, PNGDecContext *s,
         s->crow_buf          = s->buffer + 15;
         s->zstream.avail_out = s->crow_size;
         s->zstream.next_out  = s->crow_buf;
-
-        if (avctx->codec_id == AV_CODEC_ID_APNG &&
-            s->dispose_op == APNG_DISPOSE_OP_BACKGROUND)
-            memset(s->zstream.next_out, 0, s->zstream.avail_out);
     }
     s->state |= PNG_IDAT;
     if ((ret = png_decode_idat(s, length)) < 0)
@@ -887,7 +883,7 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
         pd_last += s->image_linesize;
     }
 
-    if (s->blend_op == APNG_BLEND_OP_OVER) {
+    if (s->dispose_op != APNG_DISPOSE_OP_BACKGROUND && s->blend_op == APNG_BLEND_OP_OVER) {
         uint8_t ri, gi, bi, ai;
 
         if (avctx->pix_fmt == AV_PIX_FMT_RGBA) {