cogl: Port Texture* away from CoglObject

- Make Texture a parent GObject class and move the vtable funcs as vfuncs
instead of an interface as we would like to have dispose free the TextureLoader.
- Make the various texture sub-types inherit from it.
- Make all the sub-types constructors return a CoglTexture instead of their respective
specific type. As most of the times, the used functions accept a CoglTexture,
like all the GTK widgets constructors returning GtkWidget.
- Fix up the basics of gi-docgen for all these types.
- Remove CoglPrimitiveTexture as it is useless: It is just a texture underhood.
- Remove CoglMetaTexture: for the exact same reason as above.
- Switch various memory management functions to use g_ variant instead of the cogl_ one

Note we would still want to get rid of the _cogl_texture_init which is something
for the next commit

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3193>

src/backends/meta-cursor.c

@@ -96,7 +96,7 @@ meta_cursor_sprite_clear_texture (MetaCursorSprite *sprite)
   MetaCursorSpritePrivate *priv =
     meta_cursor_sprite_get_instance_private (sprite);
 
-  g_clear_pointer (&priv->texture, cogl_object_unref);
+  g_clear_object (&priv->texture);
   meta_cursor_sprite_invalidate (sprite);
 }
 
@@ -109,9 +109,9 @@ meta_cursor_sprite_set_texture (MetaCursorSprite *sprite,
   MetaCursorSpritePrivate *priv =
     meta_cursor_sprite_get_instance_private (sprite);
 
-  g_clear_pointer (&priv->texture, cogl_object_unref);
+  g_clear_object (&priv->texture);
   if (texture)
-    priv->texture = cogl_object_ref (texture);
+    priv->texture = g_object_ref (COGL_TEXTURE_2D (texture));
   priv->hot_x = hot_x;
   priv->hot_y = hot_y;
 
@@ -264,7 +264,7 @@ meta_cursor_sprite_constructed (GObject *object)
 
   meta_cursor_tracker_register_cursor_sprite (priv->cursor_tracker, sprite);
 
-  g_clear_pointer (&priv->texture, cogl_object_unref);
+  g_clear_object (&priv->texture);
 
   G_OBJECT_CLASS (meta_cursor_sprite_parent_class)->constructed (object);
 }
@@ -276,7 +276,7 @@ meta_cursor_sprite_finalize (GObject *object)
   MetaCursorSpritePrivate *priv =
     meta_cursor_sprite_get_instance_private (sprite);
 
-  g_clear_pointer (&priv->texture, cogl_object_unref);
+  g_clear_object (&priv->texture);
 
   meta_cursor_tracker_unregister_cursor_sprite (priv->cursor_tracker, sprite);
   g_clear_object (&priv->cursor_tracker);