Workaround for lack of symmetry in Math.sin (Chrome 33 bug).

Due to a recent regression in Chrome 33, we can’t necessarily rely on Math.sin(x) being symmetric about zero, so we need some minor workarounds where it matters.

This fixes area calculations for degenerate zero-area polygons such as the one mentioned in #1753, and any potential impact this could have for the point-in-polygon algorithm (used for clipping).