Make scrollZoom speed configurable

I find scrollZoom speed painfully slow. Making this configurable may mean propagating a scale factor to some other gl modules or something, but then again it might just be a simple scalar. Maybe scrollZoomSpeed?

(Side note: it's not clear to me why scrollZoom is camelCased and belongs to config instead of to layout.)

I don't think it's entirely straightforward to get the speed itself just right across browsers and system configs, but maybe this is something: http://stackoverflow.com/questions/5527601/normalizing-mousewheel-speed-across-browsers#answer-30134826

ファイルやテストは指定されていません。まず scrollZoom の設定とブラウザにおけるホイール速度の処理を追跡し、そのうえで、ユーザー向けの速度係数がブラウザ間でどのように動作すべきかを判断してください。scrollZoom の速度を設定可能にし、一貫した動作と適切なテストカバレッジを実現できれば完了です。