Themes: If a theme only has a very-old-school 300px-wide screenshot (or narrower), don't stretch it.

props matveb.
fixes #26459.


git-svn-id: https://develop.svn.wordpress.org/trunk@26738 602fd350-edb4-49c9-b593-d223f7449a82
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.


git-svn-id: https://develop.svn.wordpress.org/trunk@26739 602fd350-edb4-49c9-b593-d223f7449a82