feat(fabric.Pattern): remove the ability to restore patterns from functions.

[asturur]

close #1621

BREAKING: patterns that uses functions are no more supported

Pattern will transition to fabric v4, but will be sort of deprectated.
A new Pattern will be created, that will allow to define patterns with fabric objects.

[sz5000]

This changes breaks fabric.PatternBrush.

brush = new fabric.PatternBrush(canvas);

... use the brush

results in an browser error

this is because this code is removed:

// function string
if (typeof fabric.util.getFunctionBody(options.source) !== 'undefined') {
this.source = new Function(fabric.util.getFunctionBody(options.source));
callback && callback(this);
}

this code is required if you use the getPatternSrc function of fabric.PatternBrush