SurfaceFlinger: Don't render rounded corners without a valid crop

A behaviour difference introduced by I6975332392756d3d96fed1d6f81245a9c7bf0a19
This will cause layers to not render if there is a corner radius
left behind.

Test: SurfaceFlinger_test
Test: repro steps in bug
Bug: 200781179
Change-Id: Ie2548fc37bb7676c10be69130d04f93e7323e79b