Stash enableCPUCores in CanvasContext

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)

frontend/document/document.lua

@@ -1,7 +1,6 @@
 local Blitbuffer = require("ffi/blitbuffer")
 local CacheItem = require("cacheitem")
 local Configurable = require("configurable")
-local Device = require("device")
 local DocCache = require("document/doccache")
 local DrawContext = require("ffi/drawcontext")
 local CanvasContext = require("document/canvascontext")
@@ -413,7 +412,7 @@ function Document:renderPage(pageno, rect, zoom, rotation, gamma, render_mode, h
     end
 
     if hinting then
-        Device:enableCPUCores(2)
+        CanvasContext:enableCPUCores(2)
     end
     self:preRenderPage()
 
@@ -429,7 +428,7 @@ function Document:renderPage(pageno, rect, zoom, rotation, gamma, render_mode, h
             logger.warn("aborting, since we do not have a specification for that part")
             -- required part not given, so abort
             if hinting then
-                Device:enableCPUCores(1)
+                CanvasContext:enableCPUCores(1)
             end
             return
         end
@@ -474,7 +473,7 @@ function Document:renderPage(pageno, rect, zoom, rotation, gamma, render_mode, h
 
     self:postRenderPage()
     if hinting then
-        Device:enableCPUCores(1)
+        CanvasContext:enableCPUCores(1)
     end
     return tile
 end