vagrant: Added CeGCC instructions
NEWS.txt: prepare version 6.5.4
SDL/Bitmap, Canvas: load mono bitmaps as grayscale

This code has been broken for a long time, ever since we switched to
PNG files and SDL_image for loading unit symbols.  libpng loads mono
images as 8-bit grayscale, and this commit changes the SDL code to
expect exactly that.
LogComputer: update comment
Driver/Volkslogger: increase bulk read timeout to 2s

Reading the flight list sometimes takes that long.
V7: Send correct value upon QNH change
LogComputer: don't stop logging after jump

After the GPS location had jumped once, the last_location attribute
was never again updated, and our IGC logger stoppedd working
(regression by 6fe91062).  The same bug was fixed by 5fdf6ba7, but
this second instance of the bug went unnoticed.

This fixes TRAC tickets #2658, #2735, #2736, #2746, #2751.
LogComputer: allow logging invalid GPS fixes

The previous commit added an extra check that is not necessary.
release v6.5.4
Merge remote-tracking branch '_max/v6.5.x' into v6.5.x