[FEATURE] Libheif update with avif support is merged!

[Wolthera]

Hey everyone,

I just merged the libheif update branch. This brings saving to
monochrome as well as 10/12 bit support to heif. Furthermore, if your
libheif is compiled with libaom or rav1e/dav1d you can save to the
shiny new avif format! For the nightly builds, amyspark has set up
rav1e/dav1d as these two are much faster than libaom, though the
quality of libaom is still superior. You can expect the quality of the
rav1e avifs to improve over time however.

What this patch also brings, and what was probably 60% of the work, is
that Krita can now generate it's own color profiles, with standard
support for most of the ITU H.273 Coding Independant Code Points
(CICP, also sometimes refered to as NCLX VUI) with the exception of
the YCbCr related code points (libheif takes care of the
transformation from YUV/YCbCr to RGB for us), and also exception for
the HDR transfer functions. For the HDR transfer functions, we convert
the images on the fly to linear, because ICC profiles right now cannot
handle these transfer functions. This is likely to change in the
future.

Anyway, special thanks to Amyspark for refactoring and handling the
libraries, Lynx3d and dmitryK for reviewing and Daniel Novomeský for
helping me figure out the mysteries of libheif.

This is a change in pigment, so it'll take a bit to build, have fun!
-- 
Wolthera