Rework GcodeModel a little towards #423

This way we can easily yield at the end of each iteration. The remaining
issue is mostly how to handle the costly numpy.fromiter, which we don't
want to do uselessly several times.

printrun/gl/libtatlin/actors.py

@@ -329,7 +329,9 @@ class GcodeModel(Model):
         prev_move_y = None
 
         prev_pos = (0, 0, 0)
-        for layer_idx, layer in enumerate(model_data.all_layers):
+        layer_idx = 0
+        while layer_idx < len(model_data.all_layers):
+            layer = model_data.all_layers[layer_idx]
             has_movement = False
             for gline_idx, gline in enumerate(layer):
                 if not gline.is_move:
@@ -446,6 +448,8 @@ class GcodeModel(Model):
             if callback:
                 callback(layer_idx + 1, num_layers)
 
+            layer_idx += 1
+
         self.count_travel_indices = count_travel_indices
         self.count_print_indices = count_print_indices
         self.count_print_vertices = count_print_vertices
@@ -682,7 +686,9 @@ class GcodeModelLight(Model):
         num_layers = len(model_data.all_layers)
 
         prev_pos = (0, 0, 0)
-        for layer_idx, layer in enumerate(model_data.all_layers):
+        layer_idx = 0
+        while layer_idx < len(model_data.all_layers):
+            layer = model_data.all_layers[layer_idx]
             has_movement = False
             for gline in layer:
                 if not gline.is_move:
@@ -707,6 +713,8 @@ class GcodeModelLight(Model):
             if callback:
                 callback(layer_idx + 1, num_layers)
 
+            layer_idx += 1
+
         self.vertices = numpy.fromiter(vertex_list, dtype = GLfloat,
                                        count = len(vertex_list))
         self.colors = numpy.fromiter(color_list, dtype = GLfloat,