Auto merge of #1108 - jrmuizel:save-flipped, r=kvark
Auto merge of #1108 - jrmuizel:save-flipped, r=kvark

Use save_flipped to save the png

THis avoids duplicating the flip code and makes the read_pixels
call match the other read_pixels calls

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servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender