qRound rounds incorrectly in certain cases

CodeChecker found a potential issue in qRound (the +0.5 method): https://clang.llvm.org/extra/clang-tidy/checks/bugprone-incorrect-roundings.html

This affects qRound at least under the default clang configuration.

Here is a test to prevent regression: https://codereview.qt-project.org/c/qt/qtbase/+/321964

However, this currently will fail. A rewrite of qRound is needed.