mesa: rename gl_client_array -> gl_vertex_array

mesa: rename gl_client_array -> gl_vertex_array

The term "client array" is a legacy thing dating back to the pre-VBO
era when _all_ vertex arrays lived in client memory.

Nowadays, it only contains vertex array state which is derived from
gl_array_attributes and gl_vertex_buffer_binding.  It's used by the
VBO module and some drivers.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>