Adjust image size to fetch for gallery view.

to comply with predefined bucket sizes, for performance/cache reasons:
https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FMultimediaViewer.git/f9e7bae91a8032fa13fc68114a0d57d190ea77f9/resources%2Fmmv%2Fmmv.ThumbnailWidthCalculator.js

Change-Id: I6fef16c591d149e5e1c7a5e7ae53eb057bbef1c3