It is used by CoverBrowser, in a subprocess, when extracting
metadata from PicDocuments, and crash on Android as calling
android methods is only allowed from the main thread.
Avoids requring Device direction in Document.
The method needs complete access to the Device object, though, so it's
just another layer of indirection, with an extra reference on the Device
object stashed in CanvasContext...
(much like it already does for Screen)
