Error in validate.CleanGeometry on fractional coordinates

[ARolek]

This is copied from tegola #338

---

The test case for this can be found here.

To run this:

• checkout the intersect_benchmark branch from my repo
• cd tegola/maths/validate
• Run: go test -v .

At this time the test is only printing results and isn't flagging the tests as failures.

Input polygon and extent:

// fractional coordinate
// if you multiply everything by 10 (remove fraction), then this test case
// works just fine.
{
	basic.Polygon { basic.Line {
			basic.Point {.1, 0},
			basic.Point {1, 0},
			basic.Point {0, 1},
	} },
	geom.NewExtent([2]float64{0, 0}, [2]float64{.5, .5}),
},

The winding order and POLYGON vs. MULTIPOLYGON aren't really relevant, the relevant part is (0.05, .5) point vs (0, 0.5) point.

Expected: POLYGON ((0.5 0,0.1 0,0.05 0.5,0.5 0.5))
Actual: MULTIPOLYGON (((0 0.5,0.1 0,0.5 0,0.5 0.5)))