lavc/pngenc: Enable frame threading for PNG encodes

Signed-off-by: Ben Jackson <ben@ben.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngenc.c

@@ -450,6 +450,7 @@ AVCodec ff_png_encoder = {
     .priv_data_size = sizeof(PNGEncContext),
     .init           = png_enc_init,
     .encode2        = encode_frame,
+    .capabilities   = CODEC_CAP_FRAME_THREADS | CODEC_CAP_INTRA_ONLY,
     .pix_fmts       = (const enum PixelFormat[]){
         PIX_FMT_RGB24, PIX_FMT_RGBA,
         PIX_FMT_RGB48BE, PIX_FMT_RGBA64BE,