Image and Gallery blocks display smaller-than-viewport images at different sizes

[benlk]

Reported in #1711:

There's a difference in how the image block and Gallery block display images that are smaller than the viewport size, with whether the images are stretched to full size:

<figure class="wp-block-image navis-slideshow navis-single navis-full" style="max-width: 100%;">
	<span class="navis-before">X</span>
	<img
		src="http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM.png"
		alt=""
		class="wp-image-1884"
		srcset="http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM.png 420w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-140x140.png 140w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-336x333.png 336w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-60x60.png 60w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-96x96.png 96w"
	>
	<figcaption>Largo is a responsive WordPress theme framework designed for news publishers. Finely-crafted and expertly-honed by the product and technology team at the Institute for Nonprofit News.</figcaption>
</figure>

<ul class="wp-block-gallery columns-1 is-cropped">
	<li class="blocks-gallery-item">
		<figure class="navis-slideshow navis-single navis-full" style="max-width: 100%;">
			<span class="navis-before">X</span>
			<img
				src="http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM.png"
				alt=""
				data-id="1884"
				data-link="http://largodev.innsandbox.wpengine.com/?attachment_id=1884"
				class="wp-image-1884"
				srcset="http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM.png 420w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-140x140.png 140w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-336x333.png 336w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-60x60.png 60w, http://largodev.innsandbox.wpengine.com/wp-content/uploads/2019/07/Screen-Shot-2019-07-17-at-2.48.12-PM-96x96.png 96w"
			>
			<figcaption>Largo is a responsive WordPress theme framework designed for news publishers. Finely-crafted and expertly-honed by the product and technology team at the Institute for Nonprofit News.</figcaption>
		</figure>
	</li>
</ul>

• Make that consistent.