Toolbar: Add a 64px crop size for the site icon.

The toolbar and post embeds request `get_site_icon_url( 64 )` and build a
2x `srcset` for high-density displays, but `WP_Site_Icon` only generated
crops at 270, 192, 180, and 32 pixels. With no 64px crop available, the
request fell back to the next largest size — the default 150x150 thumbnail —
which is far larger than needed to render a 20x20 (or 28x28 on mobile) icon.

Add a 64px entry to the list of generated crop sizes so browsers can download
a much smaller file on retina displays. This also benefits post embeds, which
request the same size.

Developed in: WordPress#12607.

Props fushar, tyxla.
Fixes #65668.

git-svn-id: https://develop.svn.wordpress.org/trunk@62823 602fd350-edb4-49c9-b593-d223f7449a82

Author: tyxla

src/wp-admin/includes/class-wp-site-icon.php

@@ -60,6 +60,9 @@ class WP_Site_Icon {
 		 */
 		180,
 
+		// High-density (2x) site icon for the admin bar and post embeds.
+		64,
+
 		// Our regular Favicon.
 		32,
 	);