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/tests/cogl/conform/test-map-buffer-range.c

@@ -25,7 +25,7 @@ vertex_data[4] =
 static void
 test_map_buffer_range (void)
 {
-  CoglTexture2D *tex;
+  CoglTexture *tex;
   CoglPipeline *pipeline;
   int fb_width, fb_height;
   CoglAttributeBuffer *buffer;
@@ -122,7 +122,7 @@ test_map_buffer_range (void)
   g_object_unref (tex_coord_attribute);
 
   cogl_object_unref (pipeline);
-  cogl_object_unref (tex);
+  g_object_unref (tex);
 
   if (cogl_test_verbose ())
     g_print ("OK\n");