Fix memory leak in GLScatterPlotItem

Fixes #103. If a ScatterPlotItem was removed from a plot and added
again, glGenTetures was called again unneccesarily. Each time it is
called, it eats up a little more space.

pyqtgraph/opengl/items/GLScatterPlotItem.py

@@ -66,6 +66,7 @@ class GLScatterPlotItem(GLGraphicsItem):
         #print pData.shape, pData.min(), pData.max()
         pData = pData.astype(np.ubyte)
         
+        if getattr(self, "pointTexture", None) is None:
             self.pointTexture = glGenTextures(1)
         glActiveTexture(GL_TEXTURE0)
         glEnable(GL_TEXTURE_2D)