[Impeller] Reorder blend filter checks to avoid unnecessary destination snapshot (flutter#42678)

Side quest of flutter#127232.

Tiny reordering to avoid unnecessary input resolution. :) Happened to notice we were hitting this case while going through some frame captures of reported fidelity issues.

Before:
![Screenshot 2023-06-08 at 1 45 48 PM](https://github.com/flutter/engine/assets/919017/aae28b7d-985d-4f16-9801-c689f8dc6f18)

After:
![Screenshot 2023-06-08 at 1 48 49 PM](https://github.com/flutter/engine/assets/919017/b667a4cd-bcf7-4e3a-98f7-0ed182b7a227)

Author: bdero

impeller/entity/contents/filters/blend_filter_contents.cc

@@ -362,21 +362,10 @@ std::optional<Entity> BlendFilterContents::CreateForegroundPorterDuffBlend(
     BlendMode blend_mode,
     std::optional<Scalar> alpha,
     bool absorb_opacity) const {
-  auto dst_snapshot =
-      input->GetSnapshot("ForegroundPorterDuffBlend", renderer, entity);
-  if (!dst_snapshot.has_value()) {
-    return std::nullopt;
-  }
-
   if (blend_mode == BlendMode::kClear) {
     return std::nullopt;
   }
 
-  if (blend_mode == BlendMode::kDestination) {
-    return Entity::FromSnapshot(dst_snapshot, entity.GetBlendMode(),
-                                entity.GetStencilDepth());
-  }
-
   if (blend_mode == BlendMode::kSource) {
     auto contents = std::make_shared<SolidColorContents>();
     contents->SetGeometry(Geometry::MakeRect(coverage));
@@ -389,6 +378,17 @@ std::optional<Entity> BlendFilterContents::CreateForegroundPorterDuffBlend(
     return foreground_entity;
   }
 
+  auto dst_snapshot =
+      input->GetSnapshot("ForegroundPorterDuffBlend", renderer, entity);
+  if (!dst_snapshot.has_value()) {
+    return std::nullopt;
+  }
+
+  if (blend_mode == BlendMode::kDestination) {
+    return Entity::FromSnapshot(dst_snapshot, entity.GetBlendMode(),
+                                entity.GetStencilDepth());
+  }
+
   RenderProc render_proc = [foreground_color, coverage, dst_snapshot,
                             blend_mode, absorb_opacity, alpha](
                                const ContentContext& renderer,