three-stage drawing: add a nodraw stage

three-stage drawing: add a nodraw stage

Normally the two stages of drawing are
1/ compute metrics of insets/rows/paragraphs/mathrow...
2/ draw the elements and cache their positions

Now the three stages are
1/ metrics
2/ nodraw: do not draw the elements, but cache their position
3/ draw the elements (and store again their position; it does not
   seems to hurt performance).

Revive the NullPainter: this replaces the setDrawingEnabled mechanism
with a painter that does nothing. The advantage is that updatePosCache
(renamed from setPosCache) does not need anymore to be invoked from
the frontend.

updatePosCache (the nodraw stage) is called at the end of
BufferView::updateMetrics.