Add jitter reduction to serial gimbal

[mmosca]

Add a deadband and possibility of skipping updates when there are no changes.

gimbal_jitter_deadband

How much position change needs to happen, before considering it different from last position. Having a value to big will decrease gimbal precision. If your controls are not precise, it is better addressed in the radio.

Default	Min	Max
0	0	20

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gimbal_jitter_min_update_rate

Try to update gimbal position at least this many times a second. (Hz) 0 only sends command if inputs changed more than +/- gimbal_jitter_deadband.

Default	Min	Max
0	0	50

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gimbal_jitter_reduction

Some gimbal reset the stabilization state when receiving a position command. When this setting is ON, only send commands to the gimbal is the position has changed.

Default	Min	Max
OFF	OFF	ON

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[mmosca]

Ideally, this should be fixed in the gimbal,not INAV.