Antialias support for line, ellipse, polygon, etc.

I'd love to add antialiasing to all drawing methods. The [`imageantialias`](http://php.net/manual/en/function.imageantialias.php) method states it works for lines and ellipses, but it doesn't support alpha components.

I would prefer a solution that works for all shapes, although it may not be possible without writing our own shape methods (prefer not to do). This may simply be a limitation of GD.

- [ ] Arc
- [ ] Ellipse
- [ ] Line
- [ ] Polygon
- [ ] Rounded rectangle
- [ ] Supports `$thickness`
- [ ] Supports `$alpha` (transparency)

Start by locating the drawing methods for arcs, ellipses, lines, polygons, and rounded rectangles, then review PHP GD's imageantialias behavior and its alpha limitations. Done means antialiasing works across the listed shapes while preserving thickness and transparency support, or the GD limitation is documented with a clearly scoped alternative.

Written by the indexing model from the issue text.