Protect critical code paths in 3D viewer

I have yet to see a crash from such an issue, but it could clearly
happen, so let's make it safe

printrun/gl/libtatlin/actors.py

@@ -20,6 +20,7 @@ import time
 import numpy
 import math
 import logging
+import threading
 
 from ctypes import sizeof
 
@@ -221,10 +222,14 @@ class Model(object):
 
     axis_letter_map = dict([(v, k) for k, v in letter_axis_map.items()])
 
+    lock = None
+
     def __init__(self, offset_x=0, offset_y=0):
         self.offset_x = offset_x
         self.offset_y = offset_y
 
+        self.lock = threading.Lock()
+
         self.init_model_attributes()
 
     def init_model_attributes(self):
@@ -341,6 +346,7 @@ class GcodeModel(Model):
         self.only_current = False
 
         while layer_idx < len(model_data.all_layers):
+            with self.lock:
                 nlines = len(model_data)
                 if nlines * 12 != vertices.size:
                     self.travels.resize(nlines * 6, refcheck = False)
@@ -505,6 +511,7 @@ class GcodeModel(Model):
                      (model_data.ymin, model_data.ymax, model_data.depth),
                      (model_data.zmin, model_data.zmax, model_data.height))
 
+        with self.lock:
             self.travels.resize(travel_vertex_k, refcheck = False)
             self.vertices.resize(vertex_k, refcheck = False)
             self.colors.resize(color_k, refcheck = False)
@@ -552,6 +559,7 @@ class GcodeModel(Model):
     # ------------------------------------------------------------------------
 
     def init(self):
+        with self.lock:
             self.layers_loaded = self.max_layers
             self.initialized = True
             if self.buffers_created:
@@ -569,6 +577,7 @@ class GcodeModel(Model):
             self.buffers_created = True
 
     def display(self, mode_2d=False):
+        with self.lock:
             glPushMatrix()
             glTranslatef(self.offset_x, self.offset_y, 0)
             glEnableClientState(GL_VERTEX_ARRAY)