GPencil: "Reproject Strokes" operator

A common problem encountered by artists was that they would accidentally move
the 3D cursor while drawing, causing their strokes to end up in weird places in
3D space when viewing the drawing again from other perspectives.

This operator helps fix up this mess by taking the selected strokes, projecting them
to screenspace, and then back to 3D space again. As a result, it should be as if
you had directly drawn the whole thing again, but from the current viewpoint instead.
Unfortunately, if there was originally some depth information present (i.e. you already
started reshaping the sketch in 3D), then that will get lost during this process.
But so far, my tests indicate that this seems to work well enough.

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -225,6 +225,10 @@ class GreasePencilStrokeEditPanel:
         if gpd:
             col.prop(gpd, "show_stroke_direction", text="Show Directions")
 
+        if is_3d_view:
+            layout.separator()
+            layout.operator("gpencil.reproject")
+
 
 class GreasePencilBrushPanel:
     # subclass must set