In the no-draw phase, do not cache the positions of not visible insets

In the no-draw phase, do not cache the positions of not visible insets

This can make a big difference for a very large branch that contains
lots of equations.

This is complementary to the previous patch, since instead of reducing
the number of calls to updatePosCache, we make it faster.

In the same test of scrolling with mouse wheel through the
branch-test.lyx document, one finds a 23% improvement for
BufferView::updateMetrics().

Part of bug #12297.