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/canvascontext.lua

@@ -35,6 +35,7 @@ The following key is required for a device object:
     * isColorEnabled() -> boolean
 ]]--
 function CanvasContext:init(device)
+    self.device = device
     self.screen = device.screen
     self.isAndroid = device.isAndroid
     self.isDesktop = device.isDesktop
@@ -45,7 +46,7 @@ function CanvasContext:init(device)
     self.hasSystemFonts = device.hasSystemFonts
     self:setColorRenderingEnabled(device.screen.isColorEnabled())
 
-    -- NOTE: Kobo's fb is BGR, not RGB. Handle the conversion in MuPDF if needed.
+    -- NOTE: At 32bpp, Kobo's fb is BGR, not RGB. Handle the conversion in MuPDF if needed.
     if device:hasBGRFrameBuffer() then
         self.is_bgr = true
         Mupdf.bgr = true
@@ -77,4 +78,8 @@ function CanvasContext:scaleBySize(px)
     return self.screen:scaleBySize(px)
 end
 
+function CanvasContext:enableCPUCores(amount)
+    return self.device:enableCPUCores(amount)
+end
+
 return CanvasContext