Fix rect shadow detection

ViewGroup_Delegate was trying to use a rect shadow even in cases where
we needed to use a path. This caused that using a ToggleButton would
throw an exception with radius < 0.
Also, changed the tests so we can detect certain render errors and
ignore other (like known fidelity warnings).

Change-Id: I900d45f9efa892ad0aebc8f4ef73fc9f21ab9d92