Resolves #5810

Changes:

• Before user setup/draw methods are called, a flag is set indicating that the preload phase is done
• After this point, load* methods will no longer increment/decrement preloads, and will no longer trigger setup getting run again

I've made an updated version of the p5 editor sketch from the issue, using a build with this new code. Previously, this would keep loading the shader again and again. Now, it just loads once: https://editor.p5js.org/davepagurek/sketches/4I3mDwb_u

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included / updated