Fix for fill artifact in WebGL mode when rendering in LINE mode using beginShape

[RandomGamingDev]

Resolves #6826

Changes:

Adds a local variable called is_line within p5.RendererGL.prototype.endShape and uses that to check whether or not to apply fill.

Screenshots of the change:

Here's the sketch demonstrating the issue (This is using the unfixed version of p5.js, but if you'd like to use the fixed version just comment out the default import for the p5.min.js import)
https://editor.p5js.org/PotatoBoy/sketches/XOezf6_IR
Without the fix:

With the fix:

PR Checklist

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

[Qianqianye]

Looks good. Thanks!