[Impeller] Enable depth buffer clipping & Stencil-then-Cover path rendering. (flutter#50856)

Turn the page, wash your hands.

Addresses the following issues:
* flutter/flutter#143077
* flutter/flutter#137714
* flutter/flutter#138460
* flutter/flutter#123671
* flutter/flutter#141961
* flutter/flutter#134432

Author: bdero

impeller/entity/contents/content_context.h

@@ -408,7 +408,7 @@ class ContentContext {
   ///
   // TODO(bdero): Remove this setting once StC is fully de-risked
   //              https://github.com/flutter/flutter/issues/123671
-  static constexpr bool kEnableStencilThenCover = false;
+  static constexpr bool kEnableStencilThenCover = true;
 
 #if IMPELLER_ENABLE_3D
   std::shared_ptr<scene::SceneContext> GetSceneContext() const;