png: allow encoding 16-bit grayscale

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/pngenc.c

@@ -255,6 +255,10 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
         bit_depth = 8;
         color_type = PNG_COLOR_TYPE_RGB;
         break;
+    case AV_PIX_FMT_GRAY16BE:
+        bit_depth = 16;
+        color_type = PNG_COLOR_TYPE_GRAY;
+        break;
     case AV_PIX_FMT_GRAY8:
         bit_depth = 8;
         color_type = PNG_COLOR_TYPE_GRAY;
@@ -460,6 +464,7 @@ AVCodec ff_png_encoder = {
     .encode2        = encode_frame,
     .pix_fmts       = (const enum AVPixelFormat[]){
         AV_PIX_FMT_RGB24, AV_PIX_FMT_RGB32, AV_PIX_FMT_PAL8, AV_PIX_FMT_GRAY8,
+        AV_PIX_FMT_GRAY16BE,
         AV_PIX_FMT_MONOBLACK, AV_PIX_FMT_NONE
     },
     .long_name      = NULL_IF_CONFIG_SMALL("PNG (Portable Network Graphics) image"),