Adding fabricImagePostProcessing callback

[Ivanca]

Adding the fabricImagePostProcessing callback so users can modify the generated Fabric.Image right before being rendered.

For example this makes masking videos using ClipPath (from fabric.js API) easy by adding a bit of code:

await editly({
  outPath: './output.mp4', 
  clips: [{
    duration: 4, layers: [
      { type: 'video', path: './assets/lofoten.mp4', cutFrom: 0, cutTo: 4 },
      { 
        type: 'video', 
        path: './assets/hiking.mp4',
        cutFrom: 0, cutTo: 4, 
        resizeMode: 'cover',
        originX: 'center', originY: 'center',
        left: 0.5, top: 0.5, width: 0.5, height: 0.5,
        fabricImagePostProcessing: async ({image, progress, canvas})=> {
          const circleArgs = {
            radius:  Math.min(image.width, image.height) * 0.4,
            originX: 'center',
            originY: 'center',
            stroke: 'white',
            strokeWidth: 22,
          };
          image.setOptions({ clipPath: new fabric.Circle(circleArgs) });
          canvas.add(new fabric.Circle({
            ...circleArgs,
            left: image.getCenterPoint().x,
            top: image.getCenterPoint().y
          }));   
        }
      }
    ]}
  ]
});

alpha.mp4

[Ivanca]

Fixed linting issues that were making the build fail, as well as deleting a console.log statement I added by mistake.

[mifi]

Very cool! I can merge this PR but I'm not sure what's the purpose of the progress argument. How would one use that? Your code does not use it

[Ivanca]

Progress helps with animation, but maybe in seconds instead of percentage would be more useful for that kind of stuff.

alpha.mp4