Make shadow sizes linear

Summary: Before, shadow size doubled with each new size until Very Large and the smaller shadows were more transparent than the larger shadows. Now shadow size increases linearly and smaller shadows start with more opacity so that they don't become nearly invisible. The new shadows should look better on cheap displays.

Test Plan:
Before
======

Small: {F6622411}
{F6632347}
Medium: {F6622412}
{F6632346}
Large: {F6622413}
{F6632344}
Very Large: {F6622415}
{F6632341}

After
=====

Small: {F6624603}
{F6632334}
Medium: {F6624606}
{F6632335}
Large: {F6624608}
{F6632336}
Very Large: {F6624612}
{F6632337}

Reviewers: #vdg, #breeze, ngraham, rooty

Reviewed By: #vdg, #breeze, ngraham, rooty

Subscribers: filipf, ngraham, zzag, rooty, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19148