Make visibility range alpha fade smoother

`smoothstep()` avoids the sudden transparency jump when entering or
leaving an object's alpha fade margin distance. This in turn helps
make opacity transitions less noticeable to the player, as it's
less likely to catch the player's eye.

servers/rendering/renderer_rd/forward_clustered/render_forward_clustered.cpp

@@ -969,10 +969,11 @@ void RenderForwardClustered::_fill_render_list(RenderListType p_render_list, con
 
 		if (inst->fade_near || inst->fade_far) {
 			float fade_dist = inst->transform.origin.distance_to(p_render_data->cam_transform.origin);
+			// Use `smoothstep()` to make opacity changes more gradual and less noticeable to the player.
 			if (inst->fade_far && fade_dist > inst->fade_far_begin) {
-				fade_alpha = MAX(0.0, 1.0 - (fade_dist - inst->fade_far_begin) / (inst->fade_far_end - inst->fade_far_begin));
+				fade_alpha = Math::smoothstep(0.0f, 1.0f, 1.0f - (fade_dist - inst->fade_far_begin) / (inst->fade_far_end - inst->fade_far_begin));
 			} else if (inst->fade_near && fade_dist < inst->fade_near_end) {
-				fade_alpha = MAX(0.0, (fade_dist - inst->fade_near_begin) / (inst->fade_near_end - inst->fade_near_begin));
+				fade_alpha = Math::smoothstep(0.0f, 1.0f, (fade_dist - inst->fade_near_begin) / (inst->fade_near_end - inst->fade_near_begin));
 			}
 		}