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/wayland/meta-wayland-single-pixel-buffer.c

@@ -119,7 +119,7 @@ meta_wayland_single_pixel_buffer_attach (MetaWaylandBuffer  *buffer,
     wl_resource_get_user_data (buffer->resource);
   uint8_t data[4];
   CoglPixelFormat pixel_format;
-  CoglTexture2D *tex_2d;
+  CoglTexture *tex_2d;
 
   if (buffer->single_pixel.texture)
     return TRUE;
@@ -143,7 +143,7 @@ meta_wayland_single_pixel_buffer_attach (MetaWaylandBuffer  *buffer,
     return FALSE;
 
   buffer->single_pixel.texture =
-    meta_multi_texture_new_simple (COGL_TEXTURE (tex_2d));
+    meta_multi_texture_new_simple (tex_2d);
 
   g_clear_object (texture);
   *texture = g_object_ref (buffer->single_pixel.texture);