add tri_incentre

src/triangle/tri_incentre.scad

@@ -0,0 +1,13 @@
+function tri_incentre(shape_pts) = 
+    let(
+        pa = shape_pts[0],
+        pb = shape_pts[1],
+        pc = shape_pts[2],
+		a = norm(pb - pc),
+		b = norm(pc - pa),
+		c = norm(pa - pb)
+    )
+	[
+        (a * pa[0] + b * pb[0] + c * pc[0]), 
+        (a * pa[1] + b * pb[1] + c * pc[1])
+    ] / (a + b + c);