feat(gles): Add GlRobustness option to disable EGL robustness

[Xavientois]

Summary

• Adds a new GlRobustness enum with Auto (default) and Disabled variants
• Adds robustness field to GlBackendOptions
• Adds environment variable support via WGPU_GL_ROBUSTNESS

Problem

On some drivers or platforms, EGL robustness context creation (EGL_EXT_create_context_robustness or EGL 1.5 core) can fail, causing adapter enumeration to fail with errors like:

Can't get wgpu adapter => No suitable adapter found:
  No suitable graphics adapter found; noop not requested, vulkan not requested, metal not requested, dx12 not requested, gl drivers/libraries could not be loaded, webgpu not requested

Solution

Add a public API option to disable EGL robustness context creation. When GlRobustness::Disabled is set, the backend skips attempting to create a robust context entirely.

Usage

Via API:

let instance = wgpu::Instance::new(&wgpu::InstanceDescriptor {
    backend_options: wgpu::BackendOptions {
        gl: wgpu::GlBackendOptions {
            robustness: wgpu::GlRobustness::Disabled,
            ..Default::default()
        },
        ..Default::default()
    },
    ..Default::default()
});

Via environment variable:

WGPU_GL_ROBUSTNESS=disabled ./your_app

Test

• cargo build succeeds
• cargo clippy --tests passes (no new warnings)
• Hard-coding robustness to disabled has worked as a fix for us at Pinterest

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This PR description was written by AI.

[inner-daemons]

Note to anyone else: this is also AI-generated in its entirety.

[Xavientois]

@inner-daemons If you'd like, I can re-add the label in the title to make it very clear to reviewers that it's AI-generated.