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.