Run screen update on new data, dump the timer.
But run an update at least once a second.
Add GPS packet to the "simple" protocol.
Make osd_pitch/roll floats to increase smoothness in AH.

We could have used int16_t, but would have to change the scale (would
have to use -32k to 32k instead of -180 to +180) which would mean more
changes in the code.
Fix build and make max. update period 500ms (needed for warnings).
Only redraw panels whose contents changed.

In every panel draw function add a variable to remember the previous
value and if it's not changed to warrant a redraw, do nothing.  This
should save us more time than any calculation optimisation becaue it
seems the SPI transfers are a bottleneck.
More robust altitude box printing.