Remove ugly/fragile special case in `view2d_masks()` that would clamp
the scrollbar-masks by the scrubbing UI. It's now possible to pass
custom scrollbar-masks to `View2D` via `UI_view2d_scrollers_draw()`. So
use this instead, making region code control its own masks, rather than
relying on special case handling in generic `View2D` code.
Also update comment in `scroller_activate_init()` to make the implicit
relationship explicit.
Alternative to, and based on the investigation in !117685.
Julian Eisel