[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