Our fancy new PhysicalModelLayer shadows are fast. :) But incorrect in some cases: #9116 #9083 #8929.

My understanding is we have a request out to Skia to provide us with an updated set of configuration parameters to match Material Design shadows with the new APIs.

Until we're confident in the correctness/fidelity of the new shadows, I think we should put them behind a flag (runtime or compile-time is fine) and disable them. (Originally noted in #8929 (comment))

@jason-simmons would you be willing to add such a flag?