Fix T67939: GPencil Noise modifier wrong random calculation

There were several problems in the old random calculation:

* Different result in the viewport and render.
* Noise "pop" in some frames. 
* Random number was calculated every time the file was opened, so get different results.

Now, instead to calculate the random numbers when n number of frames changed, the random values are calculated using a unique seed by stroke.

Also, a new Seed parameter has been added and this adds more control in the noise generated. This value can be animated and get more variations.

Differential Revision: http://developer.blender.org/D5393

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -1685,6 +1685,9 @@ class DATA_PT_gpencil_modifiers(ModifierButtonsPanel, Panel):
         row = col.row()
         row.enabled = md.random
         row.prop(md, "step")
+        row = col.row()
+        row.enabled = md.random
+        row.prop(md, "seed")
         col.prop(md, "full_stroke")
         col.prop(md, "move_extreme")