Enable captions for image widgets with custom image size

[kasparsd]

Image widgets with image size set to "custom" and custom width and height values do not render captions on the widget output.

This happens because _wp_get_image_size_from_meta() with custom as the first parameter returns false ​here and sets the $caption_size value to false which in turn causes $width to be 0 which then disables caption output during img_caption_shortcode() ​here.

[kasparsd]

Demo of the image caption missing for images with custom sizes

Trac ticket: https://core.trac.wordpress.org/ticket/50160

Use the width of a custom image size in an Image Widget instead of doing a lookup for an image size custom which doesn't have a registered width and height.

This ensures that the $width used for setting the caption width is not 0 and the captions are rendered for images with custom sizes.

[kasparsd]

Image Widget with a custom image size after applying the patch

[The one failing Travis run](​https://travis-ci.com/github/WordPress/wordpress-develop/jobs/333513693) appears to be a Travis issue.

[JavierCasares]

Tested on TwentySixteen and TwentyTwnety. The patch worked. I did not tested the Unit Test.

[audrasjb]

The pull request looks good to go (the Travis error is not related to this patch).

[SergeyBiryukov]

In 48557:

Widgets: Make sure image widgets with custom image size render captions.

Props kasparsd, JavierCasares, audrasjb.
Fixes #50160.