Angle support on macOS

[jinleili]

Connections
#2259

Compiling to Android target on macOS also triggers the (WindowKind::Unknown, Rwh::AppKit(handle)) => condition, #[cfg(not(target_os = "macos"))] was used to make the compilation pass.

Testing
Running examples on MacBook Pro(Intel Core i7, macOS 12.1), MacBook Pro(M1 Max, macOS 12.2) by WGPU_BACKEND=gl cargo run --features="angle" --example <example>, only boids example cannot runnable because of OpenGL ES 3.0:

[2022-02-06T09:12:09Z INFO  wgpu_hal::gles::adapter] SL version: OpenGL ES GLSL ES 3.00 (ANGLE 2.1.17876 git hash: f65e7c3ca739)
Using ANGLE (Apple, Apple M1 Max, OpenGL 4.1 Metal - 76.3) (Gl)
thread 'main' panicked at 'Adapter does not support the downlevel capabilities required to run this example: COMPUTE_SHADERS', wgpu/examples/boids/../framework.rs:161:5

Also tested compile to Android target on macOS through wgpu-on-app template.

No precompiled Angle libs for M1 Mac, compiled locally