API safety: [[nodiscard]] for OF types constructors

I've been subject to expecting the drawing to be red without seeing the error on a line such as ofColor(255,0,0); . C++17 has [[nodiscard]] to raise warn users of such suspicious usage. applied to constructors (such as ofColor()) it means an unused object will produce a warning. so for instance applying [[nodiscard("did you mean to use ofSetColor()?")]] to ofColor() does this:

it's pretty evident once you notice it, but depending on the context (especially when moving around languages/toolkits) it might not be apparent because it's not an error per se. similar thing can happen with ofRectangle() vs ofDrawRectangle(). maybe others, too?

proposal: strengthen the OF API by adding [[nodiscard(...)]] to constructors of types likely to be confused with a function.

(requires the -Wunused-result flag ("unused values" in Xcode); turning it on does not produce spurious issues)

Start by reviewing the API cases named in the issue, especially ofColor() versus ofSetColor() and ofRectangle() versus ofDrawRectangle(), then check how C++17 [[nodiscard]] and -Wunused-result behave in the supported builds. Done means agreeing on the constructors likely to be confused with drawing functions and documenting or applying the proposal consistently.