Is your feature request related to a problem? Please describe.
The gtk4's gl area needs us to bind to an external framebuffer to work.
See https://github.com/gtk-rs/gtk4-rs/blob/dcc7500d9b325f6047c6d74f08b0a768d0415ff2/examples/femtovg_area/femtovg_area/imp.rs#L147-L159

Hence for making it possible to display a wgpu gles image inside a gtk4 glarea, we need to provide to wgpu the glow::NativeFrameBuffer.

Describe the solution you'd like

Make it possible to add an ExternalGlFrameBuffer to the TextureInner structure so that this can be done.

Additional context
This is in substance very similar to #2609

I've succeeded in doing this here https://github.com/Doublonmousse/wgpu/tree/external-gles-framebuffer (I can open a PR if that's okay with everyone)