ImageWidget optimisations (#2394)

Avoid multiple renderings.
Don't cache big images (ie: screensaver images)

frontend/ui/widget/imagewidget.lua

@@ -82,6 +82,9 @@ function ImageWidget:_loadfile()
         if cache then
             -- hit cache
             self._bb = cache.bb
+        else
+            if self.height and self.height > 200 then -- don't cache big images
+                self._bb = Mupdf.renderImageFile(self.file, self.width, self.height)
             else
                 -- cache this image
                 DEBUG("cache", hash)
@@ -92,12 +95,16 @@ function ImageWidget:_loadfile()
                 ImageCache:insert(hash, cache)
                 self._bb = cache.bb
             end
+        end
     else
         error("Image file type not supported.")
     end
 end
 
 function ImageWidget:_render()
+    if self._bb then -- already rendered
+        return
+    end
     if self.image then
         self:_loadimage()
     elseif self.file then