Auto merge of #3344 - kvark:capture-tile, r=nical
Auto merge of #3344 - kvark:capture-tile, r=nical

Basic support for capturing tiled images

Related (but doesn't completely fix) to https://github.com/servo/webrender/issues/2236
Only ones that actually take a single tile are supported atm, and we also try not to crash on others. This is strictly an improvement over the current behavior (that just panics).

r? anyone

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