ScrolledWindow—Don’t req size for auto-hidden bars

ScrolledWindow—Don’t req size for auto-hidden bars

POLICY_AUTOMATIC means scrollbars are only shown when needed, i.e. when
the size of the window is not large enough to show the entire child. So
when measuring the preferred size, such scrollbars should be ignored.

But measure() was adding size for bars for which policy_may_be_visible()
was TRUE, which it returns for POLICY_ALWAYS (good) & _AUTOMATIC (bad).
So we reserved space for child plus scrollbars, & because we have enough
space for the child, POLICY_AUTOMATIC hides the scrollbar, leaving the
extra reserved space empty at the right/bottom sides of the child. This
is very noticeable/inconvenient for non-overlay, automatic scrollbars.

Fix this by only requesting size for scrollbars that use POLICY_ALWAYS,
rather than basing the decision on policy_may_be_visible().

https://bugzilla.gnome.org/show_bug.cgi?id=778853