gpui: Prefer removable over integrated Metal devices (#14744)

For context, see:

-
https://github.com/zed-industries/zed/issues/5124#issuecomment-2227743811
-
https://github.com/zed-industries/zed/pull/14738#issuecomment-2236613976

Short version: on Intel MacBooks it's better to prefer integrated
(`is_low_poer()`) GPUs, except when a user has an eGPU plugged-in, in
which case they very likely want to prefer that.

Before this change, we'd always prefer the integrated GPU, even if an
eGPU was available.

Now, with this change, if a user has

- eGPU
- integrated GPU
- discrete GPU

We'd first prefer eGPU, then integrated, then discrete.



Release Notes:

- Changed preference for GPUs on macOS so that eGPUs are now preferred
over integrated ones (and both of which are preferred over discrete
GPUs) on Intel Macs.