[Tizen] Re-implemented render external texture gl for impeller to avoid change FlutterOpenGLTexture and use deprecated api

[xiaowei-guan]

We have supported render external texture gl for Impeller
flutter-tizen/engine#368
But this PR has two issues:

1. It modify FlutterOpenGLTexture, it added additional variables, not a good design.

  /// The pixel data buffer.
  const uint8_t* buffer;
  /// The size of pixel buffer.
  size_t buffer_size;
  /// Callback invoked that the gpu surface texture start binding.
  BoolCallback bind_callback;
  /// The type of the texture.
  FlutterGLImpellerTextureType impeller_texture_type;

2. It uses Deprecated API

  // Deprecated: use BlitPass::AddCopy instead.
  [[nodiscard]] bool SetContents(const uint8_t* contents,
                                 size_t length,
                                 size_t slice = 0,
                                 bool is_opaque = false);

So I tried to revert offical PR to render external gl for impeller
flutter/engine#56277
But this PR doesn't work on Tizen platform, I think This PR may not have been verified.
To avoid change FlutterOpenGLTexture and use deprecated api, so I re-implemented render external texture gl for impeller based on this offical PR.
Fix flutter-tizen/embedder#131

[JSUYA]

Sorry for the late review.
I left a comment. Please check it.

[JSUYA]

@@ -138,6 +140,11 @@ sk_sp<DlImage> EmbedderExternalTextureGL::ResolveTextureImpeller(
     return nullptr;
   }

+  auto it = impeller_gl_textures_.find(texture->name);
+  if (it != impeller_gl_textures_.end()) {
+    std::shared_ptr<impeller::TextureGLES> image = it->second;
+    return impeller::DlImageImpeller::Make(image);
+  }
+
   impeller::TextureDescriptor desc;
   desc.size = impeller::ISize(texture->width, texture->height);

@@ -166,6 +173,8 @@ sk_sp<DlImage> EmbedderExternalTextureGL::ResolveTextureImpeller(
     return nullptr;
   }

+  impeller_gl_textures_[texture->name] = image;
+
   return impeller::DlImageImpeller::Make(image);
 }

If I understand correctly...
If you want to reuse a texture, we can minimize code diffs by immediately returning the image.
However, I have a few questions.

• When will the images(textures) remaining in impeller_gl_textures_ be removed ?
• Will calling impeller_gl_textures_.clear in OnTextureUnregistered() cause problems?
• Will this cause problems if the texture size changes?