Fix corner-case with transparent view visibility detect

Case where a transparent view completely overlaps a non-control (e.g. UILabel),
and visibility heuristic will determine it invisible.

Occurs in this sample app https://myshare.app.box.com/s/dm6nwjxtcko0pglpeak2

with `query("label marked:'Text 0'")`

- [ ] @jmoody please review to see if this breaks your Briar visibility tests (I remember some overlays in that suite)