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-screen-cast-stream-src.c

@@ -313,7 +313,7 @@ draw_cursor_sprite_via_offscreen (MetaScreenCastStreamSrc  *src,
   ClutterBackend *clutter_backend = meta_backend_get_clutter_backend (backend);
   CoglContext *cogl_context =
     clutter_backend_get_cogl_context (clutter_backend);
-  CoglTexture2D *bitmap_texture;
+  CoglTexture *bitmap_texture;
   CoglOffscreen *offscreen;
   CoglFramebuffer *fb;
   CoglPipeline *pipeline;
@@ -322,17 +322,16 @@ draw_cursor_sprite_via_offscreen (MetaScreenCastStreamSrc  *src,
 
   bitmap_texture = cogl_texture_2d_new_with_size (cogl_context,
                                                   bitmap_width, bitmap_height);
-  cogl_primitive_texture_set_auto_mipmap (COGL_PRIMITIVE_TEXTURE (bitmap_texture),
-                                          FALSE);
-  if (!cogl_texture_allocate (COGL_TEXTURE (bitmap_texture), error))
+  cogl_primitive_texture_set_auto_mipmap (bitmap_texture, FALSE);
+  if (!cogl_texture_allocate (bitmap_texture, error))
     {
-      cogl_object_unref (bitmap_texture);
+      g_object_unref (bitmap_texture);
       return FALSE;
     }
 
-  offscreen = cogl_offscreen_new_with_texture (COGL_TEXTURE (bitmap_texture));
+  offscreen = cogl_offscreen_new_with_texture (bitmap_texture);
   fb = COGL_FRAMEBUFFER (offscreen);
-  cogl_object_unref (bitmap_texture);
+  g_object_unref (bitmap_texture);
   if (!cogl_framebuffer_allocate (fb, error))
     {
       g_object_unref (fb);