Set sky API: Add bool for clouds in front of custom skybox

Default true.
Add 'm_clouds_enabled' bool to sky.h, set from new bool in 'set sky' API.
Make 'getCloudsVisible()' depend on 'm_clouds_enabled' instead of
'm_visible' (whether normal sky is visible).

src/sky.cpp

@@ -85,6 +85,8 @@ Sky::Sky(scene::ISceneNode* parent, scene::ISceneManager* mgr, s32 id,
 	}
 
 	m_directional_colored_fog = g_settings->getBool("directional_colored_fog");
+
+	m_clouds_enabled = true;
 }