FIX: Return `naturalWidth` and `naturalHeight` for Composer image sizes (PR #13271)

Rather than returning the size of the currently rendered image in the composer window (which is dependend on browser settings such as window size and zoom level), return the actual dimensions of the image file itself.

(Also see commit abac61449282cce61886fb1b50a3587a579e6406 which was an earlier attempt to fix this by excluding Oneboxed images entirely. That was reverted as the CSS selector didn’t work on all browsers.)