Send back unused buffers to the right pipeline

[mrobinson]

Update to latest rust-layers and send back all buffers to their
original pipeline.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3965

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[mrobinson]

@glennw Do you mind reviewing this one?

[mbrubeck]

r+

[bors-servo]

saw approval from mbrubeck
at mrobinson@cf5b619

[bors-servo]

merging mrobinson/servo/fix-buffer-return = cf5b619 into auto

[bors-servo]

mrobinson/servo/fix-buffer-return = cf5b619 merged ok, testing candidate = bc6882b

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/730
success: http://build.servo.org/builders/linux2/builds/729
success: http://build.servo.org/builders/android/builds/765
success: http://build.servo.org/builders/mac1/builds/946
success: http://build.servo.org/builders/mac2/builds/946

[bors-servo]

fast-forwarding master to auto = bc6882b