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

未指定文件或测试。首先跟踪 scrollZoom 配置和浏览器对滚轮速度的处理，然后确定面向用户的速度因子在不同浏览器之间应如何表现。完成标准是：scrollZoom 速度可配置、行为一致，并且具有适当的测试覆盖率。