RoundRect and other uses of Grow with jsRound produce incorrect result

[andersmelander]

It appears that Grow has a problem with the jsRound join style.

The following rounded rectangle was constructed with:

Grow(Rectangle(...), 1.0, jsRound, True);

[andersmelander]

Rather than trying to fix Grow I will replace it with the corresponding functionality provided by Clipper (and about time too. see #47).

[andersmelander]

Here is the same rounded rectangle using Clipper:

[andersmelander]

After updating Clipper to the newest version (AngusJohnson/Clipper2@609d873) I have measured a 5 to 10% drop in thin and thick line drawing performance.

There also appears to be some problems with mitering of line joints:

1. It looks as if the miter isn't applied if it would make the miter smaller than the line width:
   
   

2. Mitering switches on/off at MiterLimit=14.5 regardless of line width.

[andersmelander]

Above two issues are tracked in #277 and #278. Since they are minor I will merge the branch now.