optional background color value in savePixmap() settings

Fix for #274 
The way you've made this change, it only affects the background when there is a padding property on the settings object. That's probably not what we want -- any time someone specifies a background, it should be used, regardless of whether there's padding. This is especially true since the background affects the entire image, not just the padded portion.

So, I would move all of this background stuff outside of the `if (padding)` guard. Essentially, it should probably go immediately after the `args.push("rgba:-");` statement on line 126.

Note that the generator-assets plugin ALWAYS specifies padding (of 0,0,0,0 by default). So in the case of testing this with a modified generator-assets plugin, it _looks_ like it always applies.