Allow to not depend on offscreen_gl_context

[sotaroikeda]

It is created based on #773.

cc @emilio, @kvark, @glennw

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This change is 

[sotaroikeda]

A priority became higher because of Bug 1349967[1]

[1]
https://bugzilla.mozilla.org/show_bug.cgi?id=1349967

[nical]

I would prefer it if instead of adding conditional compilation all over the code we would just contain that inside of WebGLContextWrapper (and have a WebGLContextWrapper with mostly empty functions if the webrender feature isn't enabled.

[kvark]

PR is OK. I do agree with @nical that we should explore the option to cut out less stuff by providing dummies/stubs.

[kvark]

should just apply the cfg to this item only

[sotaroikeda]

Updated a patch as to reduce cfg.

[sotaroikeda]

@kvark, can you review the pull request again?

[emilio]

I guess another alternative for this would be to depend on offscreen_gl_context, but without the x11 feature (which is enabled by default).

This would make it just use the not_implemented platform by default, which would mean that you wouldn't need any cfg at all I think.

[kvark]

Oh interesting, @emilio !
@sotaroikeda would you want to try that idea out? Should be simpler.

[sotaroikeda]

It was my first proposal, but now I prefer current pull request's approach since it could make crate dependency of gecko simpler. The current approach could remove several crates from /third_party/rust of gecko.

[nical]

I like the idea of avoiding unnecessary dependencies in gecko's repository.

[emilio]

Sounds good, then this looks good to me, does anybody else have any concern with the current approach? If not, feel free to r=me.

[kvark]

Sounds like a consensus to me :)
@bors-servo r=emilio

[bors-servo]

📌 Commit 78af0cd has been approved by emilio

[bors-servo]

⌛ Testing commit 78af0cd with merge 5c2a9ff...

[bors-servo]

☀️ Test successful - status-travis
Approved by: emilio
Pushing 5c2a9ff to master...