Thrust vector are not correctly drawn

[drganjoo]

Thrust vectors being drawn seem to be incorrect as the one on the right (fr) should be:

V3D fr = quad->GetArmLength() / sqrtf(2) * quad->Attitude().Rotate_BtoI(V3F(1, 1, 0));

To test it out:

I just changed the color of front left motor to a solid red:

_glDraw->DrawArrow(pos + fl, pos + fl + down*cmd.desiredThrustsN[0]/maxThrust, V3F(1,0,0));			// front left

[arush]

I agree i found the same bug and fixed it myself before coming across this PR.