[css-transforms-2] Should the overflow area take scroll position into account.

When we have a scrolled transformed element, where the perspective property is set on the overflow:scroll element, then the accumulated transform matrix depends on scroll position (since the offset between transformed element, and perspective origin moves).

This means that the computed overflow area for the transformed element can change, and thus, the scrollable length of the overflow:scroll element (and the size of the scrollbar thumb).

The following testcase demonstrates a case where Gecko is recomputing the overflow/scrollable length as you scroll, but WebKit/blink don't, and you can't scroll the whole orange rectangle into view:
https://bug1198135.bmoattachments.org/attachment.cgi?id=8684006

You can also have the reverse case, where the transformed content scrolls fully into view early than expected, and there is remaining scroll range with no content.