[Impeller] libImpeller: Allow fetching OpenGL texture handle. (flutter/engine#55753)

Fixes https://github.com/flutter/flutter/issues/156359

cc @lyceel

engine/src/flutter/impeller/toolkit/interop/impeller.cc

@@ -544,7 +544,9 @@ ImpellerTexture ImpellerTextureCreateWithOpenGLTextureHandleNew(
     return nullptr;
   }
   texture->SetCoordinateSystem(TextureCoordinateSystem::kUploadFromHost);
-  return Create<Texture>(std::move(texture)).Leak();
+  return Create<Texture>(impeller::Context::BackendType::kOpenGLES,
+                         std::move(texture))
+      .Leak();
 }
 
 IMPELLER_EXTERN_C
@@ -557,6 +559,19 @@ void ImpellerTextureRelease(ImpellerTexture texture) {
   ObjectBase::SafeRelease(texture);
 }
 
+IMPELLER_EXTERN_C
+uint64_t ImpellerTextureGetOpenGLHandle(ImpellerTexture texture) {
+  auto interop_texture = GetPeer(texture);
+  if (interop_texture->GetBackendType() !=
+      impeller::Context::BackendType::kOpenGLES) {
+    VALIDATION_LOG << "Can only fetch the texture handle of an OpenGL texture.";
+    return 0u;
+  }
+  return TextureGLES::Cast(*interop_texture->GetTexture())
+      .GetGLHandle()
+      .value_or(0u);
+}
+
 IMPELLER_EXTERN_C
 void ImpellerDisplayListRetain(ImpellerDisplayList display_list) {
   ObjectBase::SafeRetain(display_list);

engine/src/flutter/impeller/toolkit/interop/impeller.h

@@ -493,6 +493,10 @@ void ImpellerTextureRetain(ImpellerTexture IMPELLER_NULLABLE texture);
 IMPELLER_EXPORT
 void ImpellerTextureRelease(ImpellerTexture IMPELLER_NULLABLE texture);
 
+IMPELLER_EXPORT
+uint64_t ImpellerTextureGetOpenGLHandle(
+    ImpellerTexture IMPELLER_NONNULL texture);
+
 //------------------------------------------------------------------------------
 // Color Sources
 //------------------------------------------------------------------------------

engine/src/flutter/impeller/toolkit/interop/impeller_unittests.cc

@@ -165,6 +165,9 @@ TEST_P(InteropPlaygroundTest, CanCreateOpenGLImage) {
       external_texture  //
       ));
   ASSERT_TRUE(texture);
+
+  ASSERT_EQ(ImpellerTextureGetOpenGLHandle(texture.GetC()), external_texture);
+
   auto builder =
       Adopt<DisplayListBuilder>(ImpellerDisplayListBuilderNew(nullptr));
   ImpellerPoint point = {100, 100};

engine/src/flutter/impeller/toolkit/interop/texture.cc

@@ -15,11 +15,13 @@ Texture::Texture(const Context& context, const TextureDescriptor& descriptor) {
   if (!texture || !texture->IsValid()) {
     return;
   }
+  backend_ = context.GetContext()->GetBackendType();
   texture_ = std::move(texture);
 }
 
-Texture::Texture(std::shared_ptr<impeller::Texture> texture)
-    : texture_(std::move(texture)) {}
+Texture::Texture(impeller::Context::BackendType backend,
+                 std::shared_ptr<impeller::Texture> texture)
+    : backend_(backend), texture_(std::move(texture)) {}
 
 Texture::~Texture() = default;
 
@@ -45,4 +47,12 @@ sk_sp<DlImageImpeller> Texture::MakeImage() const {
   return DlImageImpeller::Make(texture_);
 }
 
+impeller::Context::BackendType Texture::GetBackendType() const {
+  return backend_;
+}
+
+const std::shared_ptr<impeller::Texture>& Texture::GetTexture() const {
+  return texture_;
+}
+
 }  // namespace impeller::interop

engine/src/flutter/impeller/toolkit/interop/texture.h

@@ -18,7 +18,8 @@ class Texture final
  public:
   explicit Texture(const Context& context, const TextureDescriptor& descriptor);
 
-  explicit Texture(std::shared_ptr<impeller::Texture> texture);
+  explicit Texture(impeller::Context::BackendType backend,
+                   std::shared_ptr<impeller::Texture> texture);
 
   ~Texture() override;
 
@@ -34,7 +35,13 @@ class Texture final
 
   sk_sp<DlImageImpeller> MakeImage() const;
 
+  impeller::Context::BackendType GetBackendType() const;
+
+  const std::shared_ptr<impeller::Texture>& GetTexture() const;
+
  private:
+  impeller::Context::BackendType backend_ =
+      impeller::Context::BackendType::kMetal;
   std::shared_ptr<impeller::Texture> texture_;
 };