Resolves #7793

Changes:

• Instead of eagerly updating readable textures, they're updated only when accessed (either bound to a shader, or read with loadPixels() or get())

Live: https://editor.p5js.org/davepagurek/sketches/OSidpT1D1

For me in Chrome, with TEST_PERF_UPDATE=false, this sketch runs at 8fps in Firefox and 10fps in Chrome. With TEST_PERF_UPDATE=true, it hits 60 fps in both. This performance improvement scales with the size of the canvas and the number of framebuffer draws.

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated