Themes: Adjust screenshot ratios.
 * On the details overlay, always show the full 4/3 ratio.
 * On the grid, reduce to ~3/2 only if the screen width is less than 1400px. Otherwise, show the screenshot at 4/3 ratio.

props matveb, nacin.
fixes #26159.

Built from https://develop.svn.wordpress.org/trunk@26739


git-svn-id: http://core.svn.wordpress.org/trunk@26628 1a063a9b-81f0-0310-95a4-ce76da25c4cd