Customizable buffer sizes for rendered data

[Mr0grog]

Right now, the Map object always adds a 25% buffer to the data requested/rendered for each call to Map.render(). This is done to ensure that elements whose strict data doesn't overlap tiles but whose rendering does (e.g. a really fat line stroke, a large image or shield, or some long text) doesn't wind up getting visually cut off at the edge of a tile.

This is a nice convenience, but in many cases, it would be useful to be able to customize the size of the buffer area or to turn off buffering altogether (see #75). To that end, this patch adds a boundsBuffer option to both the Map constructor and to Map.render()/Map.renderGrid().

It can be:

• A number, used as a ratio of the requested image width. e.g. 0.25 means add a buffer that is 25% of the width of the rendered image.
• A function, which will be passed the requested bounds as an object and should return new bounds.

The option passed into render() will be preferred over the value set on the Map object. If no value is ever set, it will still default to 0.25.

[Mr0grog]

How does this seem to you, @prashtx, @hampelm?

[prashtx]

Nice! This looks good to me.

[hampelm]

Looks great. Thanks!

[Mr0grog]

Great, will bump versions and merge this in, then.