[next] Custom states using SPR2_SIGN display the specified skin's model instead of its signpost frame in OpenGL

This issue is most noticeable with the "old signpost" in the SRB2 The Past mod:

The root cause seems to be this chunk of code in src/hardware/hw2_md2.c:

static boolean HWR_AllowModel(mobj_t *mobj)
{
	// Signpost overlay. Not needed.
	if (mobj->state-states == S_PLAY_SIGN)
		return false;

	// Otherwise, render the model.
	return true;
}

It checks the object's state rather than its sprite2, which would probably make more sense for custom objects.