Composite physical layers on all platforms

[dnfield]

Related: #25673, #25226

We should investigate compositing physical layers on all platforms. Today, we only do it for Fuchsia.

A quick test suggests that turning off the check "just works", but it does throw an assert. Compositing physical layers on all platforms should make checking elevations and getting thicknesses more consistent and performant.

/cc @jason-simmons @mklim @goderbauer @Hixie

[Hixie]

We should check what this does to release mode performance on Android and iOS. The theory is that not having layers gives us better performance, but we never tested that for this case. We have several devicelab benchmarks that you could run locally to test this, e.g. the gallery transitions one.

[dnfield]

Worst case scenario we should be able to limit this to if asserts are enabled, if it does really turn out to help our check.

[dnfield]

This is a breaking change - for instance, framework tests currently track today whether drawShadow or clip* was called on a Canvas. With this change, those calls will no longer be there, and the test would instead have to validate whether the appropriate layer was added to the layer tree.

[dnfield]

We could, of course, avoid the breakage by making it opt in. I still have to check more performance data though - if this improves performance I'd rather not make it opt in.

[dnfield]

This had to be reverted because of anti-aliasing issues.