scrolledwindow: Only render background/frame on the widget window

Otherwise it is rendered too on each indicator window, possibly bringing
in an unintended opaque background there. Better seen with git show -b.
scrolledwindow: Avoid too frequent redraws of scrollbars

When overshooting through touchpad kinetic scrolling, the scrollbar remains
mostly static as it already hit a boundary. However, the fade in/out checks
are constantly run during scroll events, causing needless redraws in this
specific case.
mir: update surface type based on events