gles: enable seamless cube map filtering on desktop GL

[kuntal-devrat]

Connections

Fixes #10486

Description

Desktop OpenGL defaults to non-seamless cube map sampling (GL_TEXTURE_CUBE_MAP_SEAMLESS disabled), causing visible seam artifacts when filtering across cube map faces. In contrast, OpenGL ES 3.0+ and WebGL 2 mandate seamless cube map filtering by specification and don't require an enable flag, while Vulkan, Metal, and Direct3D 12 also sample cube maps seamlessly.

Since wgpu-hal requires at least OpenGL 3.3 for desktop GL, TEXTURE_CUBE_MAP_SEAMLESS (introduced in OpenGL 3.2 / GL_ARB_seamless_cube_map) is guaranteed to be supported whenever !self.shared.es.

This PR enables glow::TEXTURE_CUBE_MAP_SEAMLESS during device initialization in Adapter::open on desktop OpenGL contexts.

Testing

• Ran cargo test -p wgpu-hal --lib (passed).
• Verified cargo clippy -p wgpu-hal --all-targets --all-features (0 warnings).
• Verified cargo fmt --check and cargo xtask sync-repo-files --check.

Squash or Rebase?

Ready to rebase onto trunk.

LLM Use?

Assisted with investigating the issue, locating the initialization site in wgpu-hal, and drafting the fix and description.

Checklist

• I self-reviewed and fully understand this PR.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.
• This PR was automatically created by an Agent.
• The description was automatically written by an Agent.