Enable decoder to use more threads.
8bit YUV->RGB conversion is significantly faster
when AVIF_RGB_FORMAT_RGBA format is used,
because libavif can use libyuv to perform conversion quickly.
Prefer faster AVIF_CHROMA_UPSAMPLING_FASTEST when decoding animation.
Encoder speed changed from to 8 to 7.
Recent AV1 encoders got faster,
so it is OK to switch to better compression quality.
Remove obsolete image/avif-sequence mime-type
as it was merged with image/avif.
New libavif 0.9.1 apply some very strict standard compliance
checks by default. Unfortunately, it rejects many files made by
libheif-based apps (GIMP, ImageMagick) in the past.
libheif 1.12.0 (released on May 5, 2021) addressed the problem,
but it would be good to extend grace period, allowing other
projects and distributions to upgrade to fixed version.
Allow saving Qt-unsupported variants of ICC profiles, they could be
correctly handled by apps with wider color management support.
Change way how QColorSpace is created to avoid
rare problems in some apps.
Due to various double vs float arithmetic,
some primaries could be rejected by Qt.
If necessary, we adjust the values so they
will be accepted by Qt.
Remove newline from the ends of error strings.
