Fix garbled ImageWidget cache issues

Fixes #2805. Thanks to @poire-z, see https://github.com/koreader/koreader/issues/2805#issuecomment-296686695

frontend/document/koptinterface.lua

@@ -263,7 +263,7 @@ function KoptInterface:getCoverPageImage(doc)
     local zoom = math.min(screen_size.w / native_size.w, screen_size.h / native_size.h)
     local tile = Document.renderPage(doc, 1, nil, zoom, 0, 1, 0)
     if tile then
-        return tile.bb
+        return tile.bb:copy()
     end
 end