Avoid amp-img srcset selecting a smaller image if it will look unacceptably distorted

[acusti]

Account for proportionality ratio to srcset select calculation when choosing a smaller image over a larger one

Currently the srcset selection chooses the closest size, favoring the larger size by 10%. So if you have a 400px wide and 800px wide version of an image in the srcset, and the current <amp-img> element width is 590px (590 - 400 = 190 * 1.1 = 209; 800 - 590 = 210), amp selects the 400px version and you wind up getting a highly distorted image rendering. However, if you had 1400px and 1800px wide versions of an image, and the current element width is 1590px, choosing the 1400px version is a good call. The selection algorithm needs to account for proportionality to predict how distorted the image will wind up looking.

In this PR, I chose to make 1.2 the maximum acceptable ratio of element width to image width for choosing a smaller image. In specific terms, my first example: 590 / 400 = 1.475, which would be above the max ratio, so srcset selects the larger image. In the 2nd example: 1590 / 1400 = 1.136, ratio is below 1.2, so <amp-img> goes with the smaller image.

This also scales down well to smaller image sizes. I recently had an instance where we had a 30px and 120px wide image version in the srcset. The element was being rendered at 70px, and <amp-img> was choosing the 30px version, which of course looks unacceptable. The ratio makes it obvious: 70 / 30 = 2.33.

Related-to #1280

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[jridgewell]

LGTM, but deferring to @dvoytenko.

[acusti]

Note that I have now signed the CLA

[googlebot]

CLAs look good, thanks!

[dvoytenko]

Thanks a lot! Looks great!