Effects/Filters buffering

Currently effects/filters are not buffered so every time document is zoom in/ zoom out or object is moved all the effects are re-rendered. When there are several objects in the documents with effects applied it becomes really hard to do anything, even moving an object take a long time.

The idea is to add effects/filters buffering. When effect/filter is applied Inkscape renders it in its internal buffer with predefined dpi (this dpi could be property of document). After that this pre-rendered effect/filter can be used whenever object is moved or document is zoomed in or zoomed out (no re-rendering required).