[Bug Fix] Call uNet segmentation only after video has loaded

[mattelim]

Dear ml5 community,

I'm making a Pull Request(PR). Please see the details below.

A good PR 🌟

→ Step 1: Describe your Pull Request 📝

Fixing a Bug? Adding an Update? Submitting a New Feature? Does it introduce a breaking change?

This is small bug fix for the UNET example. The sketch previously had unstable results as the uNet segmentation function may be called before the video has loaded. The tweak moves the segmentation into a callback function videoReady of the video capture function.
video = createCapture(VIDEO, videoReady);
This pattern is copied from the BodyPix example (https://editor.p5js.org/ml5/sketches/BodyPix_Webcam).

A great PR 🌟🌟

→ Step 2: Share a Relevant Example 🦄

Here's some example code or a demonstration of my feature as a part of this pull request, a separate pull request, in the https://editor.p5js.org, or codepen/jsfiddle/etc...

Please see the live example on p5 editor:
https://editor.p5js.org/mattelim/sketches/dhNEo78-x

The best PR 🌟🌟🌟

→ Step 3: Screenshots or Relevant Documentation 🖼

Here's some helpful screenshots and/or documentation of the new feature

Previous:

Now:

[amcc]

I've also seen this issue very recently - this PR seems a good one to merge asap - seems a sensible fix to me