Add camera function
A `camera` function is convenient to draw things relative to some origin coordinates.

It should take `x, y` params and make the graphics function calls that follow (blt, pset, line, etc.) add these offets to they coordinates.  A call without parameters resets the camera.

Example in pico-8: https://www.lexaloffle.com/dl/docs/pico-8_manual.html#CAMERA (not much info there but it’s used very often in games).

Thanks!