Make IOCompositor only respond to the first Quit event.

[nnethercote]

This fixes #5234, in that the huge memory spike disappears. It still takes ~15 seconds for the window to actually disappear after that first Quit event is received by the IOCompositor. Maybe that's a pre-existing problem.

There may be better ways to do this, like handling it on the sending side (i.e. within glutin) instead of the receiving side. I just did it this way because it seemed like the easiest thing.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4280

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[jdm]

r? @glennw

[glennw]

r+

[bors-servo]

saw approval from glennw
at nnethercote@2b10f6e

[bors-servo]

merging nnethercote/servo/only-one-quit-event = 2b10f6e into auto

[bors-servo]

nnethercote/servo/only-one-quit-event = 2b10f6e merged ok, testing candidate = 2281bca

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/1029
success: http://build.servo.org/builders/linux2/builds/1029
success: http://build.servo.org/builders/android/builds/1063
success: http://build.servo.org/builders/mac1/builds/1245
success: http://build.servo.org/builders/mac2/builds/1245
success: http://build.servo.org/builders/gonk/builds/216

[bors-servo]

fast-forwarding master to auto = 2281bca