regularPointsForFrame: Allocate memory in one go (#215)

- Before
`BenchmarkPointRegularPoints-12 1476942 810.5 ns/op 360 B/op 4
allocs/op`

- After
`BenchmarkPointRegularPoints-12 2055764 548.6 ns/op 192 B/op 1
allocs/op`

Author: fx408

s2/point.go

@@ -160,12 +160,12 @@ func regularPointsForFrame(frame matrix3x3, radius s1.Angle, numVertices int) []
 	z := math.Cos(radius.Radians())
 	r := math.Sin(radius.Radians())
 	radianStep := 2 * math.Pi / float64(numVertices)
-	var vertices []Point
+	var vertices = make([]Point, numVertices)
 
-	for i := 0; i < numVertices; i++ {
+	for i := range vertices {
 		angle := float64(i) * radianStep
 		p := Point{r3.Vector{X: r * math.Cos(angle), Y: r * math.Sin(angle), Z: z}}
-		vertices = append(vertices, Point{fromFrame(frame, p).Normalize()})
+		vertices[i] = Point{fromFrame(frame, p).Normalize()}
 	}
 
 	return vertices

s2/point_test.go

@@ -458,3 +458,13 @@ func TestPointEnsureNormalizable(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkPointRegularPoints(b *testing.B) {
+	center := PointFromLatLng(LatLngFromDegrees(80, 135))
+	radius := s1.Degree * 20
+
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		regularPoints(center, radius, 8)
+	}
+}