cogl-gles2-wrapper: Fix some cases using the wrong texture unit

cogl-gles2-wrapper: Fix some cases using the wrong texture unit

At two places in cogl_wrap_prepare_for_draw it was trying to loop over
the texture units to flush some state. However it was retrieving the
texture unit pointer using w->active_texture_unit instead of the loop
index so it would end up with the wrong state.

Also in glEnableClientState it was using the active unit instead of
the client active unit.