Do not crop children to task bounds while resizing.

When we are resizing the task bounds might be smaller than
the stack bounds. We don't want to clip to the task bounds here,
so we use an expanded rectangle, making sure to expand it
large enough for giant surfaces.

Bug: 27676101
Change-Id: I1a324a474a89e4652ccd15ebd853b0b8815a48f5