Implement correct path measurement, matrix calculation, getTextAnchorShift, startOffset.

Add method and spacing attributes to textPath.
Correct startOffset calculation.
Implement method="stretch".

lib/attributes.js

@@ -114,6 +114,8 @@ const TextAttributes = {
 
 const TextPathAttributes = {
     href: true,
+    method: true,
+    spacing: true,
     startOffset: true,
     ...RenderableAttributes
 };