openxr: Remove GL readback from rendering.

[jdm]

These changes cause us to share the existing surfman device created by the webgl thread, rather than create a new one, because ANGLE refuses to share textures between different devices. In future we will want to make it possible for the webgl thread to allow the webxr device to create a surfman device, so that openxr can help us create a device with optimal settings, but this works in the short term.

[jdm]

Requires pcwalton/surfman#32.

[jdm]

TravisCI is failing because the tip of surfman is incompatible with the tip of surfman-chains (servo/surfman-chains#2) and we don't pin anything.

[bors-servo]

☔ The latest upstream changes (presumably #82) made this pull request unmergeable. Please resolve the merge conflicts.

[Manishearth]

Note that we're pinning to this on servo master, so if we rebase this we should do something to ensure the old commit sticks around. I recommend merging master into this branch

[jdm]

Isn't it enough just to not delete the branch?

[jdm]

Oh, I see what you mean. I'll open a new PR and leave this branch alone.

[Manishearth]

Yes, but I find that to usually be fragile and prone to branch deletion. We could use a tag. On Clippy usually what we do is merge in backport branches with a one-sided merge (thus adding it to the git history without causing change). In this case a `git merge master` should be enough

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