Heavily speedup print speed of exluded parts (#266)

This is achieved by reworking the logic to look at next move:
if we know the next move will set absolute E if needed, we don't need
to do it for this line. We can thus skip this G-code.

printcore.py

@@ -472,7 +472,12 @@ class printcore():
                     try: self.layerchangecb(layer)
                     except: traceback.print_exc()
             if self.preprintsendcb:
-                gline = self.preprintsendcb(gline)
+                if self.queueindex + 1 < len(self.mainqueue):
+                    (next_layer, next_line) = self.mainqueue.idxs(self.queueindex + 1)
+                    next_gline = self.mainqueue.all_layers[next_layer][next_line]
+                else:
+                    next_gline = None
+                gline = self.preprintsendcb(gline, next_gline)
             if gline == None:
                 self.queueindex += 1
                 self.clear = True

pronterface.py

@@ -417,14 +417,18 @@ class PronterWindow(MainWindow, pronsole.pronsole):
                 return True
         return False
 
-    def preprintsendcb(self, gline):
-        if self.is_excluded_move(gline):
+    def preprintsendcb(self, gline, next_gline):
+        if not self.is_excluded_move(gline):
+            return gline
+        else:
+            # Check if next move will be excluded too and if it will emit an absolute E set
+            if self.is_excluded_move(next_gline) and next_gline.e != None and not next_gline.relative_e:
+                return None # nothing to do: next move will set absolute E if needed
+            else: # else, check if this is an extrusion move with non relative E and replace it
                 if gline.e != None and not gline.relative_e:
                     return gcoder.Line("G92 E%.5f" % gline.e)
-            else:
+                else: # or just do nothing
                     return None
-        else:
-            return gline
 
     def printsentcb(self, gline):
         if gline.is_move and hasattr(self.gwindow, "set_current_gline"):