fixed the particles examples

the example was actually using a `CanvasGradient` object that is not
supported by the `me.Color` class.

this was working before for the wrong reason, and setting `fillStyle`
but without being able to set the `me.Color` `r`,`g` & `b` properties,
as `CanvasGradient` does not expose any properties and cannot be
converted to a proper string (from which to extract all color
components)

more information here :
https://developer.mozilla.org/en-US/docs/Web/API/CanvasGradient