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/koptinterface.lua

@@ -6,7 +6,6 @@ local CacheItem = require("cacheitem")
 local CanvasContext = require("document/canvascontext")
 local DataStorage = require("datastorage")
 local DEBUG = require("dbg")
-local Device = require("device")
 local DocCache = require("document/doccache")
 local Document = require("document/document")
 local FFIUtil = require("ffi/util")
@@ -111,7 +110,7 @@ function KoptInterface:waitForContext(kc)
 
     if waited or self.bg_thread then
         -- Background thread is done, go back to a single CPU core.
-        Device:enableCPUCores(1)
+        CanvasContext:enableCPUCores(1)
         self.bg_thread = nil
     end
 
@@ -392,7 +391,7 @@ function KoptInterface:renderOptimizedPage(doc, pageno, rect, zoom, rotation, re
     local cached = DocCache:check(hash, TileCacheItem)
     if not cached then
         if hinting then
-            Device:enableCPUCores(2)
+            CanvasContext:enableCPUCores(2)
         end
 
         local page_size = Document.getNativePageDimensions(doc, pageno)
@@ -425,7 +424,7 @@ function KoptInterface:renderOptimizedPage(doc, pageno, rect, zoom, rotation, re
         DocCache:insert(hash, tile)
 
         if hinting then
-            Device:enableCPUCores(1)
+            CanvasContext:enableCPUCores(1)
         end
 
         return tile
@@ -464,7 +463,7 @@ function KoptInterface:hintReflowedPage(doc, pageno, zoom, rotation, gamma, rend
     local cached = DocCache:check(hash)
     if not cached then
         if hinting then
-            Device:enableCPUCores(2)
+            CanvasContext:enableCPUCores(2)
         end
 
         local kc = self:createContext(doc, pageno, bbox)