Function combineWithFallback

combineWithFallback(pattern): Pattern
Creates a pattern that allows the fallback pattern to be painted on top of the given pattern.

Normally, a fallback pattern is only painted where no other pattern could be painted. This pattern wrapper adds an exception to this. When a pattern is wrapped with a combineWithFallback, this pattern can be painted in addition to a fallback pattern. This is useful for adding a non-filled arrowhead, as in the image example.
```
  const arrowPattern = arrow({type: ArrowType.PLAIN, size: 10, line: {width: 2}});
  const linePattern = parallelLine({line: {width: 2}});

  const regular = {
    decorations: [{
      location: 0,
      pattern: arrow
    }],
    fallback: linePattern
  };

  const combinedWithFallback = {
    decorations: [{
      location: 0,
      pattern: combineWithFallback(arrowPattern)
    }],
    fallback: linePattern
  };
```
Results in:
Parameters
- pattern: Pattern
  The pattern to combine with the fallback pattern. Cannot be null.
Returns Pattern
a pattern that allows the fallback pattern to be combined with this pattern.