Fix bounds computation when the padding is entirely due to effects and not to masking

... or something like that. I really don't have any idea what's going on here at a high level, only that there's a broken JavaScript idiom [here](https://github.com/adobe-photoshop/generator-core/blob/d37bd18052b10a7e6c001b007c17aa18c7c6cefb/lib/generator.js#L1251-L1252) for dealing with a divide-by-zero. The code in question looks like this:
```
foo = (bar / baz) || 0
```
where the disjunction is supposed to provide for a default value of zero in case `baz === 0`. The assumption is that `bar/0 === NaN` for then `NaN || 0 === 0`. But, in fact, `bar/0 === Infinity`, and `Infinity || 0 === Infinity`. After additional math, this `Infinity` turns into a `NaN` and finds its way into a `convert` parameter, which causes a `savePixmap` call to fail.

This happens to a few different assets in [this PSD](http://cloud.wehrman.org/3y33233n3i0B), but only on Windows. I don't immediately know why. It *may* be related to the fact that Photoshop seems to be returning non-integral bounds from `Generator.prototype.getPixmap` with the `boundsOnly` setting enabled; in particular, it seems to be returning a number extremely close to but not zero. (@timothynoel, is this expected!?) But that's just a totally wild guess based on my surprise at finding a float where an integer was expected. 

This bounds code is known to be crazy and in need of a rewrite, so, should I spend any more time on this now? 
![no](http://f.cl.ly/items/2l1p1s1d1r372p02401T/Photo%20Aug%2006,%2010%2018%2020%20PM.gif "no")

Conceivably, this addresses adobe-photoshop/generator-assets#259. @AlicanC if you're feeling bored and/or brave, you could let me know if this fixes your problem!

Have fun reviewing, @joelrbrandt!