[Impeller] Adjust clip coverage operations to handle per-pass clip stacks (#46912)

#46597 creates a new clip coverage stack for each subpass.  This PR updates some clip coverage operations and assertions in EntityPass::OnRender to reflect that the current clip coverage stack only contains the range of depths within the current subpass.

Author: jason-simmons

impeller/entity/entity_pass.cc

@@ -809,7 +809,8 @@ bool EntityPass::OnRender(
             ClipCoverageLayer{.coverage = clip_coverage.coverage,
                               .clip_depth = element_entity.GetClipDepth() + 1});
         FML_DCHECK(clip_coverage_stack.back().clip_depth ==
-                   clip_coverage_stack.size() - 1);
+                   clip_coverage_stack.front().clip_depth +
+                       clip_coverage_stack.size() - 1);
 
         if (!op.has_value()) {
           // Running this append op won't impact the clip buffer because the
@@ -824,20 +825,21 @@ bool EntityPass::OnRender(
           return true;
         }
 
-        auto restoration_depth = element_entity.GetClipDepth();
-        FML_DCHECK(restoration_depth < clip_coverage_stack.size());
+        auto restoration_index = element_entity.GetClipDepth() -
+                                 clip_coverage_stack.front().clip_depth;
+        FML_DCHECK(restoration_index < clip_coverage_stack.size());
 
         // We only need to restore the area that covers the coverage of the
         // clip rect at target depth + 1.
         std::optional<Rect> restore_coverage =
-            (restoration_depth + 1 < clip_coverage_stack.size())
-                ? clip_coverage_stack[restoration_depth + 1].coverage
+            (restoration_index + 1 < clip_coverage_stack.size())
+                ? clip_coverage_stack[restoration_index + 1].coverage
                 : std::nullopt;
         if (restore_coverage.has_value()) {
           // Make the coverage rectangle relative to the current pass.
           restore_coverage->origin -= global_pass_position;
         }
-        clip_coverage_stack.resize(restoration_depth + 1);
+        clip_coverage_stack.resize(restoration_index + 1);
 
         if (!clip_coverage_stack.back().coverage.has_value()) {
           // Running this restore op won't make anything renderable, so skip it.