Bug 1073290. Don't render with displayports if we're not painting to the window. r=mattwoodrow

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extra : rebase_source : 088b2156069248ac920d1a525d069b9a7330f980
Bug 1081185. Traverse rect edges when searching for w=0 crossings instead of taking diagonals. r=mattwoodrow

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extra : rebase_source : c87d145abea750cf0d8b02f0ea387ec9148e6be1