Fix various problem with temporarily unsetting background

Fix various problem with temporarily unsetting background

All the calls that unset private->parent failed if that was
not a native window (impl), instead we need to find the impl window
for the parent. Add some helper functions for this and use them.

For move/resize of child windows, we really need to recursively unset
on the parent, because moving the window could expose other native
children of the parent.

In do_shape_combine_region, only unset background if we're changing
the bounding shape (i.e. not the input shape)