Close page after screenshot to avoid memory leak

[mikaraunio]

This PR fixes a memory leak when rendering page screenshots. Renderer.screenshot() doesn't close the page on completion (even if successful), thus leaving renderer processes behind.

Follows along the lines of #268

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[mikaraunio]

I signed it!

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CLAs look good, thanks!

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[AVGP]

Looks very good to me!

[AVGP]

@mikaraunio Thank you so much for your contribution!