[effects/showfps] Calculate "correct" FPS

Summary:
cca0e15b broke code that calculates the value of the fps counter.

The frames field stores timestamps of previous frames, not time between
each two consecutive frames.

This change doesn't attempt to make the show fps compute more accurate
performance metrics, e.g. how much times it takes to execute rendering
commands on the GPU, etc.

Test Plan: The Show FPS effect displays more sensible values.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, broulik, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29247