Allow compositing in a linear color space and do so either when forcing
it via the debug controls D-Bus API, or when the experimental HDR mode
is enabled.
This relies on paint nodes etc to actually transform everything into the
linear target color space, which isn't done yet, so enabling it right
now will cause a broken result. Yet, introduce this now, so that
painting can be fixed piece by piece.
Linear blending is automatically enabled on monitors where HDR is
enabled, as this makes it possible to use an linear color space when
blending content from different color spaces with different transfer
functions.
Linear blending requires extra precision, i.e. 16 bit per channel
in the intermediate buffer due how the values are distributed,
so only enable the experimental HDR mode if the Cogl context supports
half float formats.
By default, no intermadiate linear offscreen framebuffer is used.
To test, do e.g.
./tools/debug-control.py --toggle ForceLinearBlending
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3433>
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