Fix for gif not animating when drawing to p5.graphics

[kaiserarg]

Resolves #6925

Changes:

From:
const curTime = pInst._lastRealFrameTime
To:
const curTime = pInst._lastRealFrameTime || window.performance.now();
In /image/p5.Image.js under the _animateGif(pInst) helper function

_lastRealFrameTime in main.js is set to window.performance.now() which starts the browser timer, however like the issue poster mentioned pInst._lastRealFrameTime exists on the main canvas but not graphics if you attach a gif to a graphic like below then only the first frame of the gif will animate since the helper function sets curTime to pInst._lastRealFrameTime.

`let cnv;
function preload() {
gif = loadImage('test.gif)
}

function setup() {
createCanvas(100, 100);
cnv = createGraphics(100, 100);
}

function draw() {
cnv.background(255);
cnv.image(gif, 0, 0);
image(cnv, 0, 0);
}`

In /image/p5.Image.js then under the _animateGif(pInst) curTime which is set to pInst._lastRealFrameTime will not exist (undefined) thus adding || window.performance.now(); will start the timer again allowing the gif to animate beyond the first frame.

Screenshots of the change:

PR Checklist

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

[welcome]

🎉 Thanks for opening this pull request! Please check out our contributing guidelines if you haven't already. And be sure to add yourself to the list of contributors on the readme page!

[davepagurek]

I think this makes sense. At first I was worried that this wouldn't be as robust as grabbing the time from the main canvas, but there's also the chance that the graphic is being used for fully offscreen rendering and the main canvas never updates, so this is probably the best compromise for now. We can always revisit this later, I'll merge this PR!