Gala: SSD windows have very strong shadows

[wout]

A minor issue, but it's not really pleasant on the eyes. This is the drop shadow of System Settings, which is fine:

But with Slack:

Or with Gravit Designer:

Shadows are really intense, too intense for my taste.

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[Exalm]

It's not because of Snap, but because those apps are using legacy decorations.

Here gala (well, mutter) assumes that GTK stylesheet only specifies a border in .ssd decoration style. See this line in Adwaita: https://gitlab.gnome.org/GNOME/gtk/blob/gtk-3-24/gtk/theme/Adwaita/_common.scss#L4497

elementary stylesheet doesn't do this, so mutter draws the default shadow + full box-shadow from stylesheet, so there are 2 shadows on top of each other.

[Exalm]

The stylesheet fix has landed, now it needs to be fixed in Gala.

While shadow gsettings keys indeed need to be removed, they are handy for playing with the shadows right now.

What about these vaues? It can't look 100% same because, well, the shadow is calculated differently, but it can be adjusted to look closer. The left window shows the value + the reference CSD shadow, the right one demoes the SSD shadow.

CC @danrabbit

[danrabbit]

@Exalm that's a massive improvement. Nice work! Happy to merge that