Update rust-layers to fix empty BufferRequest failures

[mrobinson]

rust-layers will now avoid sending empty buffer requests which can
cause failures when Servo tries to create empty platform surfaces.

Fixes #4060.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3254

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[jdm]

r+

[bors-servo]

saw approval from jdm
at mrobinson@737a146

[bors-servo]

merging mrobinson/servo/empty-tiles = 737a146 into auto

[bors-servo]

mrobinson/servo/empty-tiles = 737a146 merged ok, testing candidate = b33c31b

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/35
success: http://build.servo.org/builders/linux2/builds/35
success: http://build.servo.org/builders/android/builds/102
success: http://build.servo.org/builders/mac1/builds/252
success: http://build.servo.org/builders/mac2/builds/252

[bors-servo]

fast-forwarding master to auto = b33c31b