Magnetic declination West (negative value) causes incorrect heading calculation

[AlexC176]

Some more tracker testing and tuning. Using a west magnetic declination of -1556 causes an overflow of the tracker heading at approx 350 degrees. Version 11.1.0 on SPRacing hardware.

flight / imu.c file
function int16_t imuCalculateHeading(t_fp_vector *vec)

For bearings close to North (example -10) the calculation results in a int16_t overflow/underflow
return (int16_t) ((hd * 1800.0 / M_PIf) + magneticDeclination + OFFSET * 10.0f)%3600;

Suggest

float truehd = ((hd * 1800.0 / M_PIf) + magneticDeclination + OFFSET * 10.0f);
// always positive
if ( truehd < 0.0f ) { truehd = truehd + 3600.0f; )
return (int16_t) truehd%3600;

This same issue can be replicated using negative offsets. For example: magneticDeclination +10degrees (E), OFFSET -15degrees