Improve Convex Hull performance by avoiding duplicate uniquing

[dr-jts]

Improves Convex Hull performance by:

• optimizing the check for obviously degenerate inputs (those with 2 or fewer unique points)
• ensuring that scanning to extract all unique points is done only once (either via the Inner Octolateral Point Reduction heuristic, or for small inputs via a uniquing scan)

These changes can make computing the Convex Hull of large inputs more than 2x faster.