Bug: Big Sur-specific UI problems on MacOS X

[mpogue2]

Some (e.g. vertical sliders moving backwards) were fixed by moving to 5.15.2, but there is still an annoying one or two left.

1. On horizontal sliders, the scale with ticks is in light gray, and the handle is jelly-bean-shaped.
2. The singing calls highlighted colors also are misplaced vertically by about 3 pixels or so.

These could also be because Qt doesn't support SDK 11.1 yet (but I have been unable to figure out how to downgrade back to 10.15 SDK). There is a warning to this effect which says it can be suppressed, but I was unsuccessful in doing that, too.

This is a placeholder to remind me to go back and upgrade Qt again to try to remedy these.

[mpogue2]

This is all fixed in the latest Qt6.2.1 build (in the flexible_audio branch). Closing.