[effects/minimizeanimation] Make it a little bit smoother

Summary:
Don't cast translation values from double to int because it makes
animation a little bit choppy.

It makes significant difference only when animation speed is set to
"Very slow".

Test Plan:
- Go to 'System Settins > Display and Monitor > Compositor';
- Set animation speed to "Very slow"(it scales animation durations by 20x);
- Minimize/uniminimize System Settings.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14552