Don't show COG arrow if not moving.
Up to 5 decimal places in write_num().
Try using the double-speed SPI mode.
Type-safety in simple fc-link protocol.
If ground-speed / COG not available from FC, calculate it.

Not fully functional yet, needs a few fixes.
Drop printf from GS, IAS and gps-coords panels, optimise.
setHeadingPattern() only when needed, fix the func name.
Force a redraw every 10s to hide occasional Max7546 comms fails.
Drop printfs in GPSats and Heading panels.
Split setHomeVars() and only call when necessary.  Fix speed calc.

Speed calculation was missing the division by time since the travelled
distance was measured.
Also call the vars update functions in MavLink handler.