Fix T84260: NURBS edit mode lines not showing

When in edit mode, the edit lines for de-selected surfaces did not
show up.

The bug was caused by the is_gpencil bool which reused another flag.
Both grease pencil and nurbs surfaces use the edit_curve_handle shader.
A dedicated flag was added to make sure the is_gpencil bool is
set correctly.

Reviewed By: fclem

Maniphest Tasks: T84260

Differential Revision: https://developer.blender.org/D9985

source/blender/draw/intern/draw_cache_impl.h

@@ -230,6 +230,8 @@ enum {
   VFLAG_EDGE_SHARP = 1 << 6,
   VFLAG_EDGE_FREESTYLE = 1 << 7,
   /* Beware to not go over 1 << 7 (it's a byte flag). */
+  /* NOTE: Grease pencil edit curve use another type of data format that allows for this value. */
+  VFLAG_VERT_GPENCIL_BEZT_HANDLE = 1 << 30,
 };
 
 enum {