[kdecoration] Make shadows lighter

Summary:

Small

Before:

After:

Medium

Before:

After:

Large

Before:

After:

Very large

Before:

After:

With normal background

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14133