Interface changes for asymmetric stencil specification

impeller/core/formats.h

@@ -526,7 +526,7 @@ struct DepthAttachmentDescriptor {
   }
 };
 
-struct StencilAttachmentDescriptor {
+struct StencilAttachmentFaceDescriptor {
   //----------------------------------------------------------------------------
   /// Indicates the operation to perform between the reference value and the
   /// value in the stencil buffer. Both values have the read_mask applied to
@@ -546,6 +546,26 @@ struct StencilAttachmentDescriptor {
   /// Indicates what to do when both the stencil and depth tests pass.
   ///
   StencilOperation depth_stencil_pass = StencilOperation::kKeep;
+
+  constexpr bool operator==(const StencilAttachmentFaceDescriptor& o) const {
+    return stencil_compare == o.stencil_compare &&
+           stencil_failure == o.stencil_failure &&
+           depth_failure == o.depth_failure &&
+           depth_stencil_pass == o.depth_stencil_pass;
+  }
+
+  constexpr size_t GetHash() const {
+    return fml::HashCombine(stencil_compare, stencil_failure, depth_failure,
+                            depth_stencil_pass);
+  }
+};
+
+struct StencilAttachmentDescriptor {
+  // The stencil state applied to front-facing primitives.
+  StencilAttachmentFaceDescriptor front;
+  // The stencil state applied to back-facing primitives.
+  StencilAttachmentFaceDescriptor back;
+
   //----------------------------------------------------------------------------
   /// The mask applied to the reference and stencil buffer values before
   /// performing the stencil_compare operation.
@@ -558,16 +578,13 @@ struct StencilAttachmentDescriptor {
   uint32_t write_mask = ~0;
 
   constexpr bool operator==(const StencilAttachmentDescriptor& o) const {
-    return stencil_compare == o.stencil_compare &&
-           stencil_failure == o.stencil_failure &&
-           depth_failure == o.depth_failure &&
-           depth_stencil_pass == o.depth_stencil_pass &&
-           read_mask == o.read_mask && write_mask == o.write_mask;
+    return front == o.front && back == o.back && read_mask == o.read_mask &&
+           write_mask == o.write_mask;
   }
 
   constexpr size_t GetHash() const {
-    return fml::HashCombine(stencil_compare, stencil_failure, depth_failure,
-                            depth_stencil_pass, read_mask);
+    return fml::HashCombine(front.GetHash(), back.GetHash(), read_mask,
+                            write_mask);
   }
 };