St: Take the cairo fallback for large corners

The cogl path pads the corners out to the maximum corner radius to make the
math and painting logic easier. Unfortunately, when the radius exceeds the
actor's halfsize, the padding ends up interfering with other corners, creating
a big mess of rendering errors.

It'd be extremely complicated to fix this properly in the Cogl code,
so take the Cairo fallback.

https://bugzilla.gnome.org/show_bug.cgi?id=649513