getWidthFactor (shared-js)

https://github.com/readium/readium-shared-js/issues/65

https://github.com/readium/readium-js-viewer/issues/103

https://github.com/readium/readium-js-viewer/issues/105

https://github.com/readium/readium-js-viewer/issues/106

https://github.com/readium/readium-js-viewer/issues/107

https://github.com/readium/readium-js-viewer/issues/108

https://github.com/readium/readium-js-viewer/issues/109