When recording the display list for a stacking context, the following
operations (relevant to this bug) happened:
* push a stacking context
* as part of that push a None-value to the scroll frame id stack
* apply filters
* apply masking
* paint recursively
This meant that mask-images were always recorded without scroll frame
id, causing them to be painted without any scroll offset. As a result
mask-images would break as soon as the website using them was scrolled.
Instead, push to the scroll frame id stack later to solve the problem:
* push a stacking context
* apply filters
* apply masking
* push a None-value to the scroll frame id stack
* paint recursively
