FusionStyle: Some off-by-one pixel issues with high devicePixelRatio

The FusionStyle has some small issues with devicePixelRatio 2. Mostly off-by-one pixel issues resulting in 1-pixel gaps or too thin lines. Tweaking the painting code to use float based coordinates should fix these issues.

1. Tab border too thin (actually clipped, because it protrudes the widget area)
2. Tab shadow line too thin (clipped, as-well?)
3. (Only visible after fixing #1) Tab border pen needs MiterJoinStyle
4. Used border image fusion_groupbox.png needs a @2x variant. Looks washed out, otherwise.
5. Inner shadow line has a small gap
6. SpinBox separator line has a small gap
7. Scrollbar border line has a gap
8. Scrollbar slider border line has a gap
9. Frame too thin (reason for #7 and #8?)
10. gap
11. Vertical Gaps on bottom menu border line between menu items
12. Horizontal gap between menu bar and tool bar

Ptch for QRadioButton: https://codereview.qt-project.org/#/c/94136/