[whatwg] rename CanvasPixelArray

[Anne van Kesteren]

Wouldn't it make more sense to give this a more generic name, just like  
the object it is associated with? That way we can later reuse it for <img>  
elements and the like (if we want) without it having to look silly and  
poorly thought out like the rest of the platform. :-P (E.g.  
ImagePixelArray.)

--
Anne van Kesteren
<http://annevankesteren.nl/>
<http://www.opera.com/>