Fix layer selection and point translation for mouse events

Select the topmost layer at a given point to send mouse events and when
sending the event, ensure that they are relative to the layer origin,
rather than in absolute page coordinates.

Fixes #3504.
auto merge of #3538 : mrobinson/servo/events-and-layers, r=zwarich

Select the topmost layer at a given point to send mouse events and when
sending the event, ensure that they are relative to the layer origin,
rather than in absolute page coordinates.

Fixes #3504.