scrolledwindow: Ensure indicator windows are created at the right place

scrolledwindow: Ensure indicator windows are created at the right place

At the time of creating the indicator window, the scrollbar allocation is
poked and reused as the initial window dimensions. This usually happens
on two circumstances, either initially (so a ::size-allocate is emitted,
relocating the windows in the right places), or post-initialization when
calling set_overlay_scrolling() (so the scrollbars already have a valid
size allocation)

However, if the scrolledwindow is unrealized, and later re-realized again,
the scrollbars will already have a valid allocation, although 0,0 based
due to being contained in the previous indicator window. This comes out
wrong then, and the indicator window is given 0,0 based coordinates too.

Fix this by refactoring the scrollbar allocation code out of size_allocate,
and also use that given size at the time of creating the indicator windows,
this will provide the right widget-relative allocation anytime.