Bug fix for frameRate()

[quinton-ashley]

My first pr to p5.js! 🎊 This pr doesn't provide a solution to issue #6013 or add any new functionality, it just fixes a bug I found.

I saw that the _frameRate and deltaTime values were calculated after redraw() but they should be calculated before redraw()

I also avoided subtracting the same values twice by putting the deltaTime calculation first.

See @davepagurek 's example sketches to see the fix in action.

Ball's speed doesn't not match when using frameRate() vs millis():

https://editor.p5js.org/davepagurek/sketches/K7cKw7bie

With the fix the balls' speeds match:

https://editor.p5js.org/davepagurek/sketches/VXHoBahkN

[welcome]

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[davepagurek]

Thanks for making a PR for this! Before I review it more I need to do a bit more testing to make sure there aren't any other bits of code that would need to change too. I haven't combed through the code to look for anything yet but I'll keep discussing in #6013 when I do, and then I'll get back to this PR!

[davepagurek]

I finally got around to looking into what parts of p5 use frameRate and it seems like this change is safe!

I think we can merge the src/core/main.js changes, but would you mind moving the other docs change out of this one? Our usual Github flow, to help us track changes better, is to open an issue first, and then make a PR for the issue after there's been some discussion and a maintainer gives the go-ahead.

[quinton-ashley]

@davepagurek I think it worked, it says files changed (1) at the top now. You can merge now!

[davepagurek]

looks good, thanks @quinton-ashley!