Non-integer scaling factors cause flickering

Non-integer scale factors like 1.5 and 1.25 cause flickering when e.g. grid spacing is set to 1px and the window is resized. Also the grid spacing is rendered unevenly. The issue can be reproduced with the attached example with the env variable QT_SCALE_FACTOR set to 1.5. The flickering is visible with the default RHI-based rendered, with software renderer, this does not happen.

The problem can be seen on both Windows and macOS.

(Using QT_SCALE_FACTOR_ROUNDING_POLICY=Round is not an option because 125% or 150% are very frequently used on Windows and the application would use fonts too small then.)