add m_shearing

src/m_shearing.scad

@@ -0,0 +1,15 @@
+function m_shearing(sx = [0, 0], sy = [0, 0], sz = [0, 0]) = 
+    let(
+        sx_along_y = sx[0],
+        sx_along_z = sx[1],
+        sy_along_x = sy[0],
+        sy_along_z = sy[1],
+        sz_along_x = sz[0],
+        sz_along_y = sz[1]  
+    )
+    [
+        [1, sx_along_y, sx_along_z, 0],
+        [sy_along_x, 1, sy_along_z, 0],
+        [sz_along_x, sz_along_y, 1, 0],
+        [0, 0, 0, 1]
+    ];