[Tizen] Support EmbedderExternalTextureGL for impeller

[xiaowei-guan]

flutter-tizen/embedder#15
To reduce modifying the flutter engine source code, try to just add code.
Actually, we can change file "embedder_external_texture_gl" to "embedder_external_texture_gl_skia"

[JSUYA]

I thought about declaring emebedder_tizen.h and proceeded with POC, but it actually made the code more complicated. I think this PR is probably the best(?).
I was thinking in minimizing code diffs. My review may not be about code readability or efficiency.

My hope is that this commit will be cherry-picked without conflict when migrating to the next version. :)

[xiaowei-guan]

@swift-kim Do you have any suggestion?

[swift-kim]

@xiaowei-guan Nope. The current structure looks okay to me. IMO readability is more important than minimizing the code diff.

[JSUYA]

@xiaowei-guan Nope. The current structure looks okay to me. IMO readability is more important than minimizing the code diff.

Hi @swift-kim
How about adding embedder_tizen.h?
We can only add a structure dedicated to the tizen platform to the existing code and use it in user_data.
I expect it to have a significant less impact on readability. #368 (comment)
If you think that way will affect readability, we can apply this PR as is.
This PR worked well when I tested it.

[swift-kim]

I expect it to have a significant less impact on readability.

I don't quite agree on this but either way should be okay. I'm just afraid that the texture-related code is already complicated and the FlutterOpenGLTextureTizen extension will add even more complexity.

[JSUYA]

@xiaowei-guan I think, it is better to reconsider adding embedder_tizen.h by looking at conflicts that arise due to code diff when upgrading the engine and embedder later.

[xiaowei-guan]

@xiaowei-guan I think, it is better to reconsider adding embedder_tizen.h by looking at conflicts that arise due to code diff when upgrading the engine and embedder later.

Yes, we need consider this problem. I think we can also consider trying to submit the code to the official repository.

[JSUYA]

I think we can also consider trying to submit the code to the official repository.

Do you have any plan?

[xiaowei-guan]

I think we can also consider trying to submit the code to the official repository.

Do you have any plan?

This is my plan:

1. We can investigate Windows and Linux embedder code.
2. Design common interface for Windows, Linux and Tizen.
3. Create a new PR.
   But If we only submit the interface without any implementation for Windows or Linux, I don't konw whether the flutter official will agree.

[JSUYA]

It doesn't look easy. I think contributing to the official repository is a very good. I don't think you really need embedder implementations for windows and linux if you just add EmbedderExternalTextureGL. I think it's okay if there is an appropriate test case.(embedder_gl_unittests.cc)
How about you suggest EmbedderExternalTextureGL first?

[xiaowei-guan]

. I think contributing to the official repository is a very good. I don't think you really need embedder

Ok, we can try just add EmbedderExternalTextureGL for impeller. I will add test case later.

[swift-kim]

@JSUYA Contributing to the upstream is not very easy as they don't usually accept pull requests that are not used by any of the 1st-party platforms. I'm not sure if this policy has changed.