[scenes/opengl] Print a debug message when viewport limits aren't met

Summary:
This may help with debugging why compositing is suspended.

CCBUG: 418951

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28139