GaugeVario: remove useless nullptr checks from MakeAllPolygons()

These are never nullptr (false) because they are arrays.
Contest/OLCTriangle: use std::abs<int>() instead of fabs() for integer

Eliminate the useless cast from int to double and back which was
warned about by clang.
test/TestDriver: add missing <memory> include for std::unique_ptr
OrderedTask: clear active_task_point in RemoveAllPoints()

Don't leave an out-of-bounds index in active_task_point.  This can
make XCSoar crash when active_task_point>0 and the user presses "Clear
task" and then adds a new task point (TRAC #3305).
MapWindowEvents: signed integer comparison for quick zoom renderer

Bug found by clang for calling abs() on unsigned integer.