Pane opening animation should not flash the default pane backdrop

follow up to #7364

When we open a new pane there's a brief time when you can see the pane background color as the new pane slides in. This is because we immediately create the Row/Col definitions, so even if we animate the first pane getting smaller, the first pane only has half the parent grid to render into.

A solution we could do is use the background brush from the first child as the background of the parent pane, until the animation is finished. Then revert back to transparent. This will make the backgroud behind the pane sliding in look like it's the same as the first pane. This won't really help for background images, but I think this is a reasonable compromise.