DocumentRegistry: Downgrade refcount warnings to debug logging.

It can happen in perfectly sane contexts.

CReDocument: Don't destroy internal engine data when Document just
decreased the refcount (as opposed to actually tore down the document
userdata if it were the last ref).

PdfDocument: Only write edited documents if the Doc instance was torn
down.

PicDocument: Silence some DocumentRegistry related warnings

frontend/document/picdocument.lua

@@ -22,6 +22,7 @@ function PicDocument:init()
         error("Failed to open image:" .. self._document)
     end
 
+    self.is_open = true
     self.info.has_pages = true
     self.info.configurable = false