nclx: use default values that match sRGB

[kleisauke]

Since commit 062b528, libheif saves images with the default NCLX profile 6,6,6 (Rec. 601 aka NTSC) which is pretty far from sRGB and leads to incorrect colors in most viewers. Additionally, the colr HEIF box wasn't updated to reflect this.

Commit 673fb03 resolves the latter issue by guaranteeing the update of target_nclx_profile following the color conversion, thereby ensuring the propagation of any defaults specified in ColorConversionPipeline::convert_image().

Commit 3bc7572 resolves the former issue by ensuring NCLX profiles defaults to 1,13,1 (Rec. 709 with IEC 61966-2-1 transfer function).

Context: libvips/libvips#3731

Thanks to @DarthSim for discovering this.

[novomesk]

I agree that NCLX with unspecified 2/2/2 should not be interpreted as 6/6/6.

I understand that default interpretation should be sRGB but I believe it should be 1/13/6 and not 1/13/1. (BTW I also use 1/13/1 in my software but it must be explicitly set that way).

I think it is OK to leave initial NCLX values to 2/2/6 - matrix coefficients is good to define, but primaries and transfer function could be defined via ICC.

[farindk]

I've implemented the handling of unspecified values in a different way that avoids overwriting the target_nclx_profile in de41052.
Combined with your changes of the default values, I think this should solve the problem.

[kleisauke]

Thanks! I can confirm that PR #1017 resolves this.