Improve gallery sizing

[westonruter]

This is intended to resolve issues reported with galleries in https://wordpress.org/support/topic/classic-block-galleries-amp-issues/

The appearance of galleries became unpredictable when images in landscape and portrait appear in the same gallery. The amp-carousel dimensions were being obtained from the max height of all images and the max width of all the images. Since the amp-carousel had a responsive layout, the width/height resulted in unexpected letterboxing above or to the side of the images. So this PR seeks to make the galleries more consistent by always opting for showing the carousel so that there will be no letterboxing above/below the image. Landscape images will fill the carousel, and portrait images will be constrained to fit inside the landscape box.

This PR also supplies missing srcset for images in shortcode galleries.

Testing

Create a gallery with two images, one portrait and one landscape:

• https://via.placeholder.com/1367x2048
• https://via.placeholder.com/2048x1367

Supply the attachment IDs in post_content such as the following (taking the place of 2715,2714):

<!-- wp:heading -->
<h2>Gallery Block</h2>
<!-- /wp:heading -->

<!-- wp:gallery {"ids":[2715,2714],"ampCarousel":true} -->
<ul class="wp-block-gallery columns-2 is-cropped" data-amp-carousel="true"><li class="blocks-gallery-item"><figure><img src="https://wordpressdev.lndo.site/content/uploads/2019/07/1367x2048-684x1024.png" alt="" data-id="2715" data-link="https://wordpressdev.lndo.site/?attachment_id=2715" class="wp-image-2715"/><figcaption>First slide</figcaption></figure></li><li class="blocks-gallery-item"><figure><img src="https://wordpressdev.lndo.site/content/uploads/2019/07/2048x1367-1024x684.png" alt="" data-id="2714" data-link="https://wordpressdev.lndo.site/?attachment_id=2714" class="wp-image-2714"/><figcaption>Second slide</figcaption></figure></li></ul>
<!-- /wp:gallery -->

<!-- wp:heading -->
<h2>Gallery Shortcode</h2>
<!-- /wp:heading -->

<!-- wp:shortcode {"ampCarousel":true} -->
[gallery ids="2715,2714"  link="none" size="large"]
<!-- /wp:shortcode -->

<!-- wp:heading -->
<h2>Gallery in Classic Block</h2>
<!-- /wp:heading -->

<p>[gallery link="none" size="large" ids="2715,2714"]</p>

Before

Before Portrait

Before Landscape

After

After Portrait

After Landscape

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Build for testing: amp.zip (v1.2.1-beta1-20190722T201452Z-11d51844)

[westonruter]

@rsmith4321 Please test the amp.zip build on your site.