Add missing `glTransformFeedbackVaryings`

[rmri]

Based on the documentation it seems this could be available for use in OpenGLES 3.0:
https://www.khronos.org/registry/OpenGL-Refpages/es3.0/html/glTransformFeedbackVaryings.xhtml

This would be very useful to create particle fields, let me know if you could add this.
Thank you.

[zhaolei]

glTransformFeedbackVaryings done. but not test 😄

https://developer.mozilla.org/en-US/docs/Web/API/WebGLTransformFeedback

[rmri]

@wasabia I'm using pub to include the project.
Do you think a version update is needed for these changes to take effect?
Maybe bumping to 0.0.16? If there's another way to update let me know, I'm a bit new to Flutter.

Thanks.

[zhaolei]

clone this repo to your local then you can use local pub

like flutter_gl example project

https://github.com/wasabia/flutter_gl/blob/main/flutter_gl/example/pubspec.yaml

[rmri]

@wasabia Thanks.

Tested, I think we need to use Int8 pointers and cast the native utf8 to int8 like this:

/// OpenGLContextES.dart
transformFeedbackVaryings(program, count, List<String> varyings, bufferMode) {
    final varyingsPtr = calloc<Pointer<Int8>>(varyings.length);

    int i = 0;
    for (final varying in varyings) {
      varyingsPtr[i] = varying.toNativeUtf8().cast<Int8>();
      i = i + 1;
    }
    final result = gl.glTransformFeedbackVaryings(program, count, varyingsPtr, bufferMode);
    calloc.free(varyingsPtr);

    return result;
  }

Instead of:

flutter_gl/flutter_gl/lib/openGL/opengl/OpenGLContextES.dart

Lines 813 to 826 in 21de2e5

 transformFeedbackVaryings(program, count, List<String> varyings, bufferMode) { 
   final varyingsPtr = calloc<Pointer<Utf8>>(varyings.length); 
  
   int i = 0; 
   for(final varying in varyings) { 
     varyingsPtr[i] = varying.toNativeUtf8(); 
     i = i + 1; 
   } 
  
   final result = gl.glTransformFeedbackVaryings(program, count, varyingsPtr, bufferMode); 
   calloc.free(varyingsPtr); 
  
   return result; 
 } 

I will test more and maybe set up a fork and send a PR.