Fix editor save generating unusable levels without sprites.

The argument inversion mistake causing the problem was introduced with
the removal of the package manager. fs::relative() takes the arguments
the other way round than fs_relative(). fs_relative() should be
removed to reduce this confusion...

tsc/src/level/level.cpp

@@ -959,7 +959,7 @@ bool cLevel::Joy_Button_Up(unsigned int button)
 
 fs::path cLevel::Get_Music_Filename() const
 {
-    return fs::relative(pResource_Manager->Get_Game_Music_Directory(), m_musicfile);
+    return fs::relative(m_musicfile, pResource_Manager->Get_Game_Music_Directory());
 }
 
 void cLevel::Set_Music(fs::path filename)

tsc/src/level/level_background.cpp

@@ -205,7 +205,7 @@ void cBackground::Set_Image(const fs::path& img_file_1)
 
     // Make the path relative to pixmaps/ if it isn’t yet
     if (m_image_1_filename.is_absolute())
-        m_image_1_filename = fs::relative(pResource_Manager->Get_Game_Pixmaps_Directory(), m_image_1_filename);
+        m_image_1_filename = fs::relative(m_image_1_filename, pResource_Manager->Get_Game_Pixmaps_Directory());
 
     Clear_Images();
     Add_Image_Set("main", m_image_1_filename);

tsc/src/objects/sprite.cpp

@@ -516,7 +516,7 @@ xmlpp::Element* cSprite::Save_To_XML_Node(xmlpp::Element* p_element)
     // Only save the relative part of the filename -- otherwise the
     // generated levels wouldn’t be portable.
     if (img_filename.is_absolute())
-        img_filename = fs::relative(pResource_Manager->Get_Game_Pixmaps_Directory(), img_filename);
+        img_filename = fs::relative(img_filename, pResource_Manager->Get_Game_Pixmaps_Directory());
 
     Add_Property(p_node, "image", img_filename.generic_string());
 
@@ -1453,7 +1453,7 @@ void cSprite::Editor_Activate(void)
 
     p_editor->Add_Config_Widget(UTF8_("Image"), UTF8_("Image filename"), editbox);
 
-    fs::path rel = fs::relative(pResource_Manager->Get_Game_Pixmaps_Directory(), m_start_image->Get_Path());
+    fs::path rel = fs::relative(m_start_image->Get_Path(), pResource_Manager->Get_Game_Pixmaps_Directory());
     editbox->setText(path_to_utf8(rel));
     editbox->subscribeEvent(CEGUI::Editbox::EventTextChanged, CEGUI::Event::Subscriber(&cSprite::Editor_Image_Text_Changed, this));
 

tsc/src/video/animation.cpp

@@ -1157,7 +1157,7 @@ void cParticle_Emitter::Set_Image_Filename(const fs::path& filename)
     // remember the filename for saving
     m_image_filename = filename;
     if (filename.is_absolute())
-        m_image_filename = fs::relative(pResource_Manager->Get_Game_Pixmaps_Directory(), filename);
+        m_image_filename = fs::relative(filename, pResource_Manager->Get_Game_Pixmaps_Directory());
 
     // set new image
     Set_Image(pVideo->Get_Surface(m_image_filename, 0));