Add support for GL_EXT_stencil_wrap by guessing that the skip of two values in

Add support for GL_EXT_stencil_wrap by guessing that the skip of two values in
the register header was meaningful.  It turns out those were the proper values
for the plain INCR/DECR ops, while what we were using as INCR/DECR were the
_WRAP versions.  Tested with stencil_wrap (didn't expose normal/_WRAP swapping)
and stencilwrap (exposed it nicely) tests.  Props to idr for poking me about
adding this.