vbo/save: avoid dangling_attr_ref situation

vbo/save: avoid dangling_attr_ref situation

dangling_attr_ref=true can be set when the following happens:

  glBegin(GL_TRIANGLES)
  glVertex(...)
  glVertex(...)
  glColor4(...)
  glVertex(...)

When glColor4 is hit, the first 2 vertices are copied to the vertex_store
by upgrade_vertex, but since this is done before glColor4 new values are
copied, we make a note to fixup these attribute laters using dangling_attr_ref.

This causes very slow rendering. What this commit does instead, is in this
situation, the new attribute value are backported to the vertex store for the
copied vertices after upgrade_vertex is done updating the layout.

This avoids the slow corner case.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7912

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20495>