fix blurry widgets on high DPI displays

Consider display scaling factor when setting up low-level image buffers (QPixmap and QImage) in some scope widgets

Without setting displayPixelRatio those surfaces look blurry on hidpi screens when scaling factor is set in the OS. Adjusted it on all affected widgets I could find as well as the Splash screen.

You can notice it in the screenshots below when looking at the Volume scale of the monitor or the min/max values in the RGB parade widget.

PS: I have no idea what I'm doing (first time c++ and QT) but surprisingly, looks like only minor adjustments are necessary so I thought I may be able to fix/finish this issue as it's bugging me for quite some time 🤞 So I'd appreciate even the most obvious feedback 😀

PS: Could only test it on Linux/Arch, would appreciate if someone could test it on Windows/Mac when it's ready 🙏

PS: Good job on the README / developer documentation 👍 . Probably spent more time, figuring out this KDE Invent gitlab setup than running and compiling kdenlive.

before (blurry)

after (sharp)