TraceComputer: activate contest trace with a small number of points

The contest trace will be used for the triangle solver. The number of
points is limited by the computing power, because the triangle solver's
runtime can be up to about O(n^2.2).

Assuming Win CE devices are slow is safe.
TaskComputer: use contest trace for triangle solver
OLCTriangle: reset branch and bound tree if n_points < 3
OLCTriangle: check tree size after pruning

Fixes a segfault in some cases
OLCTriangle: make algorithm interruptible

This finally fixes lock-ups on slow devices. It distributes the calculation
over several UpdateIdle() calls. Tests showed that a complete run can take
up to 20 ticks, but sometimes finishes within one single tick.

One limitation is that this currently works only for predictive runs
(e.g. assuming the trace will be closed later), otherwise some more
state-saving must be done. XCSoar itself does only predictive solving,
only SkyLines needs the non-predictive code so far.

Fixes #2780