Tiled Gallery: Smoother resize.

This PR tries to make resizing more responsive, so that it happens constantly and immediately. This will also prevent images from overflowing to the next row on square tiles.

Resizing works quite smoothly on Chrome when only resizing on animation frames. However, on Firefox and Safari (and others) this causes an annoying overflow to the next row for square and circle tiles. For these browsers it seems completely ok to just resize whenever there is an event without any lag introduced.

I've tried IE 10 (with changes from https://github.com/Automattic/jetpack/pull/499 for better compatibility) as well as latest Firefox, Safari, Opera, Chrome and Chrome Canary (as is here in the PR). On all of these resizing seems a lot smoother: no overflow to next row, almost immediate (in IE 10, though running in a VM might have something to do with it) or immediate resize of images visible.

Thoughts?