target_par::gvthres is hard-coded to 4 cameras

[yosefm]

It should be replaced by a dynamically-allocated array, like in the other parameter structs that need to know the number of cameras. This would require adding a new_target_par function and possibly also checking for buffer overflow errors in functions that take the camera number (i.e., that the given number is less than the actual number of cameras).

On second thought, these checks should be introduced anyway.