Make winit truly optional

[kvark]

Fixes #64

[kvark]

This looks straightforward (and @grovesNL is travelling)
bors r=travis

[bors]

Build failed

• continuous-integration/travis-ci/push

[kvark]

Alright, this is nasty. So when we have a feature that enables "xxx/yyy", where "xxx" is optional, this automatically enables "xxx" out of a sudden. Anybody knows a workaround? @omni-viral hit this before in Amethyst, perhaps?

[zakarumych]

I hit it multiple times. There is an issue in cargo (I'd insert link here, but can't find it). The issue was created long ago, but no one fixed it. Same as issue where dependencies that are filtered out by platform still enable features in other dependencies. We need a hero!

Intermediate solution is listing dependencies that can't be build on any platform under target.'cfg(...)'.dependencies. It will fix building. It would include more backends than necessary, but this should affect building times only, which is better than non-compilation anyway.

For gfx backends I write it like this:

[dependencies]
gfx-hal = "0.3"
gfx-backend-empty = { version = "0.3", optional = true }
gfx-backend-gl = { version = "0.3", optional = true }

[target.'cfg(all(target_os = "windows", not(target_arch = "wasm32")))'.dependencies]
gfx-backend-dx12 = { version = "0.3", optional = true }

[target.'cfg(all(any(target_os = "windows", all(unix, not(any(target_os = "macos", target_os = "ios")))), not(target_arch = "wasm32")))'.dependencies]
gfx-backend-vulkan = { version = "0.3", optional = true }

[target.'cfg(any(all(target_os = "macos", not(target_arch = "wasm32"), all(target_arch = "aarch64", target_os = "ios"))))'.dependencies]
gfx-backend-metal = { version = "0.3", optional = true }

And I have a macro that repeat those cfgs for code that uses backend specifics.

[kvark]

I decided to go a different route and just route the responsibility to the user of the library (which shouldn't be a problem for wgpu-rs, and possibly easy for others?..). Also filed #7259.

[seivan]

I suspect this is a typo.

[kvark]

@seivan thank you for the quick review! Apparently, there was a bunch of code pieces using the old "window-winit" that are now rewritten.

[kvark]

Finally, CI appears to be happy.
bors r=seivan

[bors]

Build succeeded

• continuous-integration/travis-ci/push

[zakarumych]

@kvark This can be real problem because cargo enables features eagerly. User won't be able to enable features based on target platform and thus would end up reexport them same way transitively to the binary. And features would have to be enabled to build final binary.