Introduce DevicePixel and use it for layer boundaries

This will mean that Servo can use rust-layers without ambiguity to the
coordinate system of variables and arguments.
Use DevicePixels for TileGrid size

This is the next step in making all units in rust-layers typed.
Store tile_size and calculate BufferRequests in DevicePixels
Store content_offset using typed units
Merge pull request #102 from mrobinson/typed-units

 Start using typed units in rust-layers