Fix definition file for TypeScript 1.6

[DanielRosenwasser]

TypeScript 1.6 will be making a breaking change where object literals with excess properties will cause an error. As part of a sweep through DefinitelyTyped (microsoft/TypeScript#4081), I found that pixi's tests were causing errors, though the only errors I see in the tests are

Error TS2345 Argument of type '{ backgroundColor: number; }' is not assignable to parameter of type 'RendererOptions'.
  Object literal may only specify known properties, and 'backgroundColor' does not exist in type 'RendererOptions'.

I added a backgroundColor property to RendererOptions to fix this, but if there are any others that are missing, we should add them to avoid breaks in consumer code.

[clark-stevenson]

Awesome @DanielRosenwasser I do not know where you find the time!

I have added a review to my todo this weekend to check for any other rogues.

Thanks

[DanielRosenwasser]

You're welcome! Do you generally sync up the changes to DefinitelyTyped, or would it be easier if I took care of it?

[clark-stevenson]

@bfanger only recently made a PR to push to DT at #14

I am really far behind with all the Definitely Typed stuff so he may be able to answer more credibly than me.