Correct math to blend color in the blur effect

Summary:
Signed-off-by: André Agenor <andreagenor@icloud.com>

This is the old way blur effect work
{F6718496}

Now we have better blend colors
{F6718498}

Reviewers: tcanabrava, patrickelectric

Reviewed By: tcanabrava

Tags: #kde_applications

Differential Revision: https://phabricator.kde.org/D20036