Add a new curve type for elliptical arcs
and use it for rounded rectangles and circles.
We use the 'E' command to represent elliptical
arcs in serialized paths.
This commit adds the basic infrastructure for paths.
The public APIs consists of GskPath, GskPathPoint and
GskPathBuilder.
GskPath is a data structure for paths that consists
of contours, which in turn might contain Bézier curves.
The Bezier data structure is inspired by Skia, with separate
arrays for points and operations. One advantage of this
arrangement is that start and end points are shared
between adjacent curves.
A GskPathPoint represents a point on a path, which can
be queried for various properties.
GskPathBuilder is an auxiliary builder object for paths.
