Make IsVertexArray() return false before BindVertexArray()

Make IsVertexArray() return false before BindVertexArray()

Rename existing _Used flag to EverBound.

The GL 4.3 and ES 3.0 specs say

   These names are marked as used, for the purposes of GenVertexArrays
   only, but they do not acquire array state until they are first bound.

This also affects Apple VAOs, which is fine since the
APPLE_vertex_array_object spec says

   A vertex array object is created by binding an unused name. This
   binding is accomplished by calling BindVertexArrayAPPLE with id set
   to the name of the new vertex array object.

Fixes arb_vertex_array_object_isvertexarray.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>