If not in air-mode the mixer is not able to apply positive boosting
and roll_pitch_scale is recomputed to apply symmetric - reduced - thrust.
This has the consequence to cut completely the outputs when the thrust is
set to zero.
Align the way motor output is computed with the model used in the mixer generation script.
previous:
`out = (roll_gain*roll + pitch_gain*pitch + yaw_gain*yaw + thrust)*out_gain`
new:
`out = roll_gain*roll + pitch_gain*pitch + yaw_gain*yaw + thrust_gain*thrust`
which is the standard matrix*vector multiplication.
This commit has 0 effect on symmetric platforms (i.e. most) as thrust_gain==1 on these platforms.
On asymmetric platform --where some thrust_gain are lower than 1-- the previous formulation resulted in coupling between thrust and other axes.
fixes#8628
Beat Küng