GPv3: Draw Tool: Make "Simplify" option a screen space threshold

Previously, the "Simplify" option was a world space distance threshold.
This meant that zooming in and out of the view changed the way
this option behaved. There were complaints from artists about this.

This change improves two things:
* The simplify algorithm uses the screen space coordinates rather than the
  3D positions.
* The UI setting is in pixels making it much easier to tweak (no need
  for values in the 1e-4 range).

Pull Request: https://projects.blender.org/blender/blender/pulls/122719

scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -2735,7 +2735,7 @@ class VIEW3D_PT_tools_grease_pencil_v3_brush_post_processing(View3DPanel, Panel)
         col1.prop(gp_settings, "pen_subdivision_steps", text="Subdivisions")
 
         col1 = col.column(align=True)
-        col1.prop(gp_settings, "simplify_factor")
+        col1.prop(gp_settings, "simplify_pixel_threshold", slider=True)
 
         col1 = col.column(align=True)
         col1.prop(gp_settings, "use_trim")