mesa: Introduce a yet unused _DrawVAO.

During the patch series this VAO gets populated with either the currently
bound VAO or an internal VAO that will be used for immediate mode and
dlist rendering.

v2: More comments about the _DrawVAO, filter and enabled mask.
    Rename _DrawVAOEnabled to _DrawVAOEnabledAttribs.
v3: Fix and move comment.

Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/context.c

@@ -1335,6 +1335,8 @@ _mesa_free_context_data( struct gl_context *ctx )
 
    _mesa_reference_vao(ctx, &ctx->Array.VAO, NULL);
    _mesa_reference_vao(ctx, &ctx->Array.DefaultVAO, NULL);
+   _mesa_reference_vao(ctx, &ctx->Array._EmptyVAO, NULL);
+   _mesa_reference_vao(ctx, &ctx->Array._DrawVAO, NULL);
 
    _mesa_free_attrib_data(ctx);
    _mesa_free_buffer_objects(ctx);