Allow rust-layers to stop caring about the Servo content scale

[mrobinson]

These commits allow rust-layers to store all layer boundaries in device/layer coordinates instead of Servo page coordinates. Additionally, BufferRequests are now returned in device/layer coordinates as well. Servo will take care of converting them to page coordinates.

[zwarich]

Is rust-layers actually going to need to use the content_offset? I assume so, since it needs to render scrolled layers at the right position.

[mrobinson]

Yes, rust-layers will need two types of transformations per layer. The content_offset is used to adjust the position of tiles within the layer area.

[zwarich]

I guess my question is: if rust-layers isn't using content_offset yet, does it need to be added in this PR? Or does rust-layers need to expose it so that Servo can use it before rust-layers can use it?

[mrobinson]

Yes, exactly right. rust-layers exposes it so that it can be used in get_buffer_requests_recursively in Servo. In a later patch I will move get_buffer_requests_recursively entirely into rust-layers.