Refactor background image, fix fixed attachment. Improve acid2.
Convert a method from &JSRef to JSRef

This also removes the unnecessary formation of a trait object.
auto merge of #3535 : glennw/servo/fixed-bg, r=pcwalton
auto merge of #3536 : zwarich/servo/more-jsref, r=Manishearth

This also removes the unnecessary formation of a trait object.
Use LayerPixel for Layer bounds and most arguments

When interacting with Layers it is simpler to use LayerPixels, which
are unscaled pixels in the Layer coordinate system. This removes a lot
of room for error and makes things simpler.
auto merge of #3537 : mrobinson/servo/scale, r=zwarich

When interacting with Layers it is simpler to use LayerPixels, which
are unscaled pixels in the Layer coordinate system. This removes a lot
of room for error and makes things simpler.