Make sgrb-trc color profile be the default for GrayAU8 color space
It fixes loading/saving of PNG images (and the
corresponding unittest).
WARNING: This patch also makes linear profile to be a default for
GrayAF16, GrayAU16 and GrayAF32 color spaces. Theoretically,
it can break loading some file formats. I have already fixed
PNG and TIFF. What else can load GrayA16+?