The magnitude of the noise in the final output scales with:
1.0 - cogl_color_out.a
A fixed value for the magnitude relative to cogl_color_out.a might be
too small for large cogl_color_out.a or too large for small
cogl_color_out.a (or possibly even both).
Compensate for this to achieve constant magnitude in the final output.
The coefficients were determined experimentally as a compromise between
preventing colour banding with dark background content and tolerable
noise with bright background content.
This helps for the issue described in
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7795 .
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3572>
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