mesa: keep the current VAO also set as _DrawVAO

mesa: keep the current VAO also set as _DrawVAO

This removes setting _DrawVAO from all glDraw* functions that draw
primitives because now _DrawVAO is always equal to the current VAO.

glEnd, glRasterPos, and the slow display list path now have to save
_DrawVAO before changing it, and restore it after they are done.

This also removes _EmptyVAO, which becomes unused because we no longer
bind "no VAO" into _DrawVAO.

Rules:
- only glBindVertexArray and non-glDraw* paths should call
  _mesa_set_draw_vao, other places shouldn't
- only draws should call _mesa_update_vao_state, other places shouldn't

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19953>