deprecate ofBackground or refactor to mean ofClear

this suggestion comes from a discussion with @ofzach last week.

right now we have a few functions for working with the background in OF:

1. ofClear(): clears the background color right away
2. ofBackground(): clears the background color right away, and saves the background color
3. ofSetBackgroundColorAuto(): enables or disables whether the saved background color is used to clear the screen at the beginning of each frame
4. ofSetBackgroundColor(): changes the saved background color without clearing the screen

having so many functions creates confusion. the only reason that anything besides ofClear() exists is because processing sets an example of putting the background() call in setup() as if it is a property of the document rather than an action.

i suggest we only use ofClear(). alternatively, we can change the name of ofClear() to ofBackground() and note in the changelog that it doesn't work in setup() anymore.

Start by reviewing the four background-related functions named in the issue: ofClear(), ofBackground(), ofSetBackgroundColorAuto(), and ofSetBackgroundColor(). Check their current usage and the discussion referenced in the issue. Done means the API direction is decided and the corresponding deprecation or rename, behavior changes, and changelog updates are agreed and implemented.