Tweak default ProceduralSkyMaterial and PhysicalSkyMaterial appearance

- Tweak colors to be less saturated and more balanced (in terms of hue).
  The cool blue sky is balanced by a warm brown ground,
  which makes reflections look closer to how they'd look like when using
  an HDRI panorama texture.
- Make the ground color dark on both ProceduralSkyMaterial and
  PhysicalSkyMaterial to reduce indoor light leaking, especially
  when using GI.
- Tweak the PhysicalSkyMaterial colors to be as close as possible
  to ProceduralSkyMaterial (with the default sun orientation).
- Tweak editor environment defaults to be identical to the default
  ProceduralSkyMaterial colors. Previously, the default editor sky
  color was different from the colors of a newly created
  ProceduralSkyMaterial resource.

Both new skies were tested without GI, with SDFGI and with VoxelGI.
They were tuned to look best when using ACES tonemapping with a
whitepoint set to 6, but they still look good with other
tonemapping operators.

editor/plugins/node_3d_editor_plugin.cpp

@@ -7353,8 +7353,8 @@ void Node3DEditor::_load_default_preview_settings() {
 	sun_angle_altitude->set_value(-Math::rad2deg(sun_rotation.x));
 	sun_angle_azimuth->set_value(180.0 - Math::rad2deg(sun_rotation.y));
 	sun_direction->update();
-	environ_sky_color->set_pick_color(Color::hex(0x91b2ceff));
-	environ_ground_color->set_pick_color(Color::hex(0x1f1f21ff));
+	environ_sky_color->set_pick_color(Color(0.385, 0.454, 0.55));
+	environ_ground_color->set_pick_color(Color(0.2, 0.169, 0.133));
 	environ_energy->set_value(1.0);
 	environ_glow_button->set_pressed(true);
 	environ_tonemap_button->set_pressed(true);