Update get/toJSON/loadJSON methods with focalOffset

[nikhilxb]

• Mirror other properties (target, position, etc.) by adding getFocalOffset().
  • Useful because sometimes setFocalOffset() values need to be calculated as a function of the current values, which are changed during trucking.
• Preserve focalOffset, focalOffset0 in toJSON()/loadJSON().
  • Needed for completeness for recreating camera state.

[nikhilxb]

Note that this could be a breaking change, because loadJSON() currently does not seem to use default values in case obj.focalOffset/obj.position/etc do not exist. So it is not possible to use loadJSON() with JSON saved from earlier versions.

But this problem is not specific to focalOffset, as any adding any new field in the future will result in the same problem. Not sure if it matters to you at this stage, just thought I'd bring it up.

[yomotsu]

Thanks for your first contribution!
Looks perfect!

If you could, would you mind adding getFocalOffset to the doc(readme.md) as well?

Note that this could be a breaking change

I will manage that later👍

[nikhilxb]

Done!

[yomotsu]

Thank you so much!

[yomotsu]

just merged