Fix #142691: Grease Pencil: Reset Vertex Color obstructs materials

The default value for the `vertex_color` attribute is `0.0f` not `1.0f`
so the `Reset Vertex Color` operator should set zero, to no obstruct
the base material.

Candidate for backport to 4.5 LTS.

Pull Request: https://projects.blender.org/blender/blender/pulls/146277

source/blender/editors/grease_pencil/intern/grease_pencil_vertex_paint.cc

@@ -444,7 +444,7 @@ static wmOperatorStatus grease_pencil_vertex_paint_reset_exec(bContext *C, wmOpe
           info,
           use_selection_mask,
           [&](const ColorGeometry4f & /*color*/) -> ColorGeometry4f {
-            return ColorGeometry4f(1.0, 1.0, 1.0, 1.0);
+            return ColorGeometry4f(0.0f, 0.0f, 0.0f, 0.0f);
           });
       any_changed.store(any_changed | changed, std::memory_order_relaxed);
       return;