Fix T102126 Regression: Grease Pencil: Broken 2D Layering

This was because `stroke_id` was not using `vertex_start`.
But since `vertex_start` is not 1 based like it used to be, we need to add
1 to it to avoid a fragment depth of `0.0` which would be equal to the
background and not render.

source/blender/draw/intern/draw_cache_impl_gpencil.cc

@@ -319,7 +319,7 @@ static void gpencil_buffer_add_point(GPUIndexBufBuilder *ibo,
 
   vert->strength = (round_cap0) ? pt->strength : -pt->strength;
   vert->u_stroke = pt->uv_fac;
-  vert->stroke_id = gps->runtime.stroke_start;
+  vert->stroke_id = gps->runtime.vertex_start;
   vert->point_id = v;
   vert->thickness = max_ff(0.0f, gps->thickness * pt->pressure) * (round_cap1 ? 1.0f : -1.0f);
   /* Tag endpoint material to -1 so they get discarded by vertex shader. */