add a wrapper for UpdateCamera

[kiran-kp]

• UpdateCamera expects a pointer so it can update the data
  • The wrapper is a macro so that we can modify the lisp variable
• Add example
• Fixes #15

[kiran-kp]

I still think it's worth writing a define-struct macro to simplify writing the wrappers but I didn't want to make this change too large.

[longlene]

Looks great! Thank you!