Getting rid of macros and dead code:

Summary:

• convert defines and macros to constexpr variables and functions
• remove experimental code which was introduced in 55dd369cb24375369633518e5ce260fbd6d72688, and unused since 61df57e5b9cc34ba9104f6d62add89ec933c00ee
• remove dead DEBUG_LAYOUT code, which wouldn't compile since commit efb0f40b7b16fb70d32cecd91dd8d4d9decd61c5, as that deleted a function used there

fix: do a comparision in Q_ASSERT, not an assignment

Reviewers: KDE Games, aacid

Reviewed By: aacid

Subscribers: aacid, KDE Games

Differential Revision: https://phabricator.kde.org/D11814