mesa: reference shared state in glPushAttrib(GL_TEXTURE_BIT)

mesa: reference shared state in glPushAttrib(GL_TEXTURE_BIT)

This fixes a dangling texture object pointer bug hit via wglShareLists().
When we push the GL_TEXTURE_BIT state we may push references to the default
texture objects which are owned by the gl_shared_state object.  We don't
want to accidentally delete that shared state while the attribute stack
references shared objects.  So keep a reference to it.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>