cbindgen stumbles over egl type aliases

[jimblandy]

The following code in wgpu-hal/src/gles/egl.rs triggers a bug in cbindgen:

#[cfg(not(feature = "emscripten"))]
type EglInstance = egl::DynamicInstance<egl::EGL1_4>;

#[cfg(feature = "emscripten")]
type EglInstance = egl::Instance<egl::Static>;

As described in mozilla/cbindgen#286, cbindgen stumbles when the same name refers to a generic type in one module and a non-generic type in another, and then a type alias is introduced for one in the scope of the other. (As the docs warn, cbindgen doesn't really understand Rust modules).

To reproduce, run cbindgen --lang C in the wgpu-hal directory (using cbindgen version 0.19.0). For me, this panics:

thread 'main' panicked at 'Instance is not generic', src/bindgen/ir/opaque.rs:105:9

This is obviously a cbindgen bug, not wgpu's problem. But comments in the cbindgen issue linked above ominously suggest that this is a consequence of mozilla/cbindgen#7, which seems like a pretty difficult architectural issue to fix. So I think it makes sense for wgpu to work around it.