Implement fill-opacity on Path objects with gradients.

[ajvincent]

This fixes the smaller issue in #5810 . That said, I'm not sure I put the test in the right place. It took me longer than I would've liked just to craft a test that works.

Please review with a bit of caution, and if the test is in the wrong place, please provide a better test for the right file. (I'm still quite an amateur at the fabric.js codebase.)

[asturur]

As soon this is corrected and merged i can look in the geometry problem.

[ajvincent]

I am uncomfortable with this latest patch, because I have changed the API to fabric.Gradient.fromElement: specifically, the second argument has become the third, and the opacity attribute has become the second attribute. Feel free to reject (again) if you would rather the new opacity attribute be appended to the end of the arguments to preserve API.

[asturur]

I agree that would be better to preserve the argument order, even if fromElement is supported just for internal use for the whole SVG parser.
But I really cannot predict if someone is using it stand alone, and if they managed succesfully would be nice to do not break the API for them.

[asturur]

i will add you svg example as a visual test ( that will probably fail for now )