Implement Canvas pixel manipulation

[jdm]

Rebase of #4639.

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4060

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[jdm]

r+

[bors-servo]

saw approval from jdm
at jdm@325400d

[bors-servo]

merging jdm/servo/canvas = 325400d into auto

[bors-servo]

jdm/servo/canvas = 325400d merged ok, testing candidate = 287f390

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/828
success: http://build.servo.org/builders/linux2/builds/826
success: http://build.servo.org/builders/android/builds/862
success: http://build.servo.org/builders/mac1/builds/1043
success: http://build.servo.org/builders/mac2/builds/1043
success: http://build.servo.org/builders/gonk/builds/14

[bors-servo]

fast-forwarding master to auto = 287f390