`vello` should disable the `wgpu` gles backend by default

[nicoburns]

Vello Classic does not work with this backend so it would be better if we did not compile it in.

[DJMcNab]

I think this turns into two issues:

• Our accursed millstone of vello::utils, when doing adapter selection, should make sure to not select gles.
• Our wgpu_default feature should manually write out the default features, excluding gles. I don't much like this proposal, because it:
  • Would mask any issues which arose around (1), because if a user compiled in gles, we could start failing.
  • It means we have to keep up with wgpu's default features. Failure to do so has a really nasty failure mode, in that if we miss an important default feature, the user has to manually add a wgpu dependency, and enable that feature themselves. But from a practical perspective, we're very unlikely to going to remember to do so.

From a practical perspective, a user who cares about this can take on that work themselves; they have a more holistic view of their entire application, so if they have a compilation failure, they can just enable the default features themselves.