Fix default preview environment sky.

Fixes the horizon color being grey even when both
the sky and ground colors are set to black, and
makes the energy slider affect both hemispheres
rather than only affecting the top "sky" portion.

editor/plugins/node_3d_editor_plugin.cpp

@@ -8534,8 +8534,10 @@ void Node3DEditor::_preview_settings_changed() {
 	}
 
 	{ //preview env
-		sky_material->set_sky_energy_multiplier(environ_energy->get_value());
-		Color hz_color = environ_sky_color->get_pick_color().lerp(environ_ground_color->get_pick_color(), 0.5).lerp(Color(1, 1, 1), 0.5);
+		sky_material->set_energy_multiplier(environ_energy->get_value());
+		Color hz_color = environ_sky_color->get_pick_color().lerp(environ_ground_color->get_pick_color(), 0.5);
+		float hz_lum = hz_color.get_luminance() * 3.333;
+		hz_color = hz_color.lerp(Color(hz_lum, hz_lum, hz_lum), 0.5);
 		sky_material->set_sky_top_color(environ_sky_color->get_pick_color());
 		sky_material->set_sky_horizon_color(hz_color);
 		sky_material->set_ground_bottom_color(environ_ground_color->get_pick_color());