The scrollbar painter has a bit of code rot.

While working on #13253, I found it hard to follow the painter logic and found several little bugs. This really needs a clean up to make it easier to work with. Some of the bugs I found have to do with padding and margins, these should likely be wrapped in consistent getters to use across the all of the painter methods.

Found bugs:

• Scrollbar margins are handled inconsistently #106918
• RawScrollbar does not apply track or border color #106917
• Scrollbar track offset does not account for padding #106834

Fix is in #107179