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>
