Engine/Task/Ordered/OrderedTask: Clear taskpoint_start/taskpoint_finish when clearing a task

Fixes trac issue #2947.
MapWindowEvents: check if the projection is valid

Fixes assertion failure when the DrawThread is slow on startup.
OLCTriangle: better approximation of min and max distance

Reduces the number of needed iterations by 40% and memory
consumption by 30% in some cases.
OLCTriangle: use a mixed breadth-first/depth-first search for large trees

This won't affect fast problems but can reduce the memory usage for hard
ones.

Experiments with worst-case flights with many gps fixes revealed a
saving of over 2/3 compared to the old strategy (450mb instead of 1.6gb
for a flight with 4096 points). Flights with less points don't have that
problem, due to a nonlinear dependency to flight size.
OLCTriangle: reduce memory footprint of CandidateSet

Saves about 10% memory...
Hardware/DisplayGlue: remove unused variable on Koob
Screen/Util: move code to IsCircleVisible()

IsCircleVisible() contains a simplified version of the code, without
temporary PixelRect objects.
Screen: remove unused function SetRect()
Event/Notify: enable the destructor on Kobo
Event/SDL: include cleanup
Event/{Console,Android}/Queue: add Push(Callback)

Similar API to the SDL EventQueue.
Event/SDL: make EventQueue a class

Be API-compatible with the other EventQueue implementations.
Event/SDL/Queue: add methods Pop(), Wait()

Move code from Loop.cpp.
Event/SDL: don't use SDL_WaitEvent()

Prepare for TimerQueue.
Event/SDL: use TimerQueue instead of SDL_INIT_TIMER

SDL_INIT_TIMER uses a dedicated thread for timers.  That's pretty
complicated.  But the biggest problem is that SDL is not thread-safe,
and may crash at any time.
Added turn_rate_heading_smoothed to CirclingInfo

LP filtered heading turn rate is needed for some thermalling features and
infoboxes.
Driver/LX: support Nano firmware 2.10

The Nano protocol was changed after firmware 2.05.  This is some more
fallout of TRAC #2819.
Android/InternalGPS: detect when GPS is lost on Android 2.3 and older

Fix for TRAC #2929.
Driver/FLARM: verify checksums of answers

This attempts to improve the reliability of task declarations.
release v6.6.3