pngdec: set correct range

AV_PIX_FMT_GRAY8/16 are considered YUV formats, and the color_range is
not set - so the API user will have to assume limitted range. (Unless
the API user adds a special-case for the PNG decoder.)

Just export the correct range - full range.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pngdec.c

@@ -633,6 +633,8 @@ static int decode_idat_chunk(AVCodecContext *avctx, PNGDecContext *s,
             return AVERROR_INVALIDDATA;
         }
 
+        avctx->color_range = AVCOL_RANGE_JPEG;
+
         if ((ret = ff_thread_get_buffer(avctx, &s->picture, AV_GET_BUFFER_FLAG_REF)) < 0)
             return ret;
         ff_thread_finish_setup(avctx);

tests/ref/fate/png-ya16

 #tb 0: 1/25
-0,          0,          0,        1,    49152, 0x93ce1cd5
+0,          0,          0,        1,    49152, 0x0801ce78

tests/ref/fate/png-ya8

 #tb 0: 1/25
-0,          0,          0,        1,    49152, 0xed73bf03
+0,          0,          0,        1,    49152, 0x5a1481f8