ports/cef: Implement accelerated compositing for the CEF port.

[pcwalton]

@glennw, is it possible to rebase your stuff on top of this? Sorry for the mess.

r? @metajack

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3354

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[glennw]

@pcwalton No worries, I'll rebase on this once it lands - the Linux changes are minor.

[metajack]

@pcwalton Comments in critic. This will also need a rebase.

[pcwalton]

re-r? @metajack (just one issue left which is resolved in my other PR, so if it's OK I'd like to address it there and land this)

[pcwalton]

re-r? @metajack

[metajack]

@pcwalton Finished re-r. 3 remaining issues, all small.

[metajack]

Ready for squash.

[metajack]

@pcwalton: build failed

window.rs:10:89: 10:108 error: unresolved import `compositing::windowing::FinishedWindowEvent`. There is no `FinishedWindowEvent` in `compositing::windowing`
window.rs:10 use compositing::windowing::{ScrollWindowEvent, ZoomWindowEvent, NavigationWindowEvent, FinishedWindowEvent};
                                                                                                     ^~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Could not compile `glut_app`.