Windows: Fix regression introduced by eliminating use of OpenGL ES 3.1 symbols

[jnschulze]

Fixes flutter/flutter#102925

This fixes a regression introduced in #32780 (by changing GL_RGBA8 to an unsized GL_RGBA).

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[cbracken]

lgtm! Thanks!

[Hixie]

shouldn't this be tested?

[jnschulze]

shouldn't this be tested?

I‘m not sure.
The existing tests in fact don‘t cover the FlutterOpenGLTexture.format.
But just comparing the format against a known value wouldn‘t add a lot of value here.
If there had been a test, @cbracken would have updated it as well as he was confident that the new pixel format is fine.

So we might need more sophisticated tests actually spinning up the engine and making it use a texture to see if Skia accepts it.
I will look into that.