Fix and cleanup used filament length measurement

gcoder.py

@@ -303,17 +303,17 @@ class GCode(object):
         cur_e = 0
         
         for line in self.lines:
-            if line.command() == "G92":
             if line.e != None:
-                    total_e += cur_e
+                continue
+            if line.command() == "G92":
                 cur_e = line.e
-            elif line.is_move() and line.e:
+            elif line.is_move():
                 if line.relative_e:
-                    cur_e += line.e
+                    total_e += line.e
                 else:
+                    total_e += line.e - cur_e
                     cur_e = line.e
 
-                
         return total_e