AP_Camera: 2nd camera should use different PWM output

[rmackay9]

Similar to #23164, if two camera servo backends are setup (e.g. CAM1_TYPE=1, CAM2_TYPE=1) then they should use different PWM outputs. Currently they will both output to any SERVOx_FUNCTION = 10 (CameraTrigger).

There are two possible solutions:

1. create a new servo function called something like "Camera2Trigger" or "CameraTrigger2"
2. enhance SRV_FUNCTION library so that a caller can get just the X instance of the servo output. Then camera1 could use the first instance (e.g. the one with the lower servo output) and camera2 could use the 2nd.

This issue was brought up on this 4.5 discussion https://discuss.ardupilot.org/t/separate-cam1-and-cam2-triggering/117468

[Ryanf55]

Would you like a multi-cam gazebo simulator set up to develop this against, or do you have hardware capable of doing it?

[rmackay9]

Hi @Ryanf55,

Thanks for the kind offer! I think it's actually fairly straight forward to test even without the real hardware though.