External gles framebuffer

[Doublonmousse]

Connections

Fixes #7581

Description
Allow the gles backend to bind to an framebuffer.

Testing

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[cwfitzgerald]

Looking good, some small comments

[cwfitzgerald]

Thanks!