When loading a symbolic SVG, the original file is wrapped in
a header that includes a generated stylesheet with the
appropriate colors, plus an overall opacity.
The opacity value is clamped to values between 0 and 1, which
directly matched the alpha field of the GdkRGBA type. However
since we moved to StIconColors/ClutterColor, the alpha field
is now in the range [0..255], any alpha value other than 0 now
results in full opacity.
To fix, translate the [0..255] integer into a [0..1] double
before using it as opacity.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2731>
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