Expose DEFAULT_OFFSET from Camera

[bampakoa]

Closes #2599

[hpinkos]

Thanks for the pull request @bampakoa! We got your CLA emails and we'll try to review this next week.

[emackey]

Can we easily explain to users the difference between this and Camera.DEFAULT_VIEW_FACTOR? How does a user decide which one to use?

(Don't get me wrong, I like this change, I just want to protect the usability of the API).

[bampakoa]

@hpinkos Do I need to elaborate further more on the documentation, as @emackey refers, in order for the PR to be reviewed?

[emackey]

@bagnell When you get a minute, what do you think of this?

[bagnell]

@bampakoa I think this is great.
@emackey You can merge this whenever its ready.

[emackey]

Thanks @bampakoa. I'll add a note in CHANGES.md in master.