MainWindow: clear the bottom_widget attribute in KillBottomWidget()

Fixed use-after-free bug.
Add polar for Pilatus B4 at the end of the list.
This polar was measured in 1974 at the Idaflieg-vergleichsfliegen at Aalen.
Permission is granted by Idaflieg (via e-mail) to use this polar in XCSoar.
test/src/TaskPrinting.cpp: prevent assert fails on verbose output
test/src/harness_waypoints.cpp: prevent aliasing of wp
test/src/test_replay_olc.cpp: make functional again after refactor

Code refactoring in IgcReplay and elsewhere broke this test.
Some tests fail now but at least it runs and passes some tests.
test_debug: add waypoint file in command line options
Merge branch 'v6.6.x'
SDL/Color: remove obsolete #ifndef
po: update Italian translation
po: update Polish translation
OpenGL/Color: add internal typedef Component

Merge some of the OpenGL vs GLES code.
Screen/Color: convert to class

Hide internal attributes by default.
OpenGL/Color: optimise method WithAlpha()

Don't export/import the r/g/b components.  Construct new Color object
with unmodified internal values.
OpenGL/Color: implement GLfixed tolerance in IsOpaque()
TopographyFileRenderer: use Color::IsOpaque()
New contest solver "Retrospective"

This maintains a short list of way-points, that summarises the flight.
Not hooked up to anything yet.
test setup for retrospective replay
dlgAnalysis/FlightStatisticsRenderer: draw retrospective task

On OLC page, it's useful to see place names summarising where we've been.
This is currently the only place where the retrospective task functionality
is reported.

Possibly it should use a different font, and be centered etc.
test/plot/plotretro: add waypoint labels to plot
Solvers/Retrospective: use angle differences for pruning