cogl-texture-2d: Flush the journal before mipmapping

In the case of indirect rendering like the first frame to use mutter's
background wallpaper:

  Texture_A -> FBO_B (Texture_B) -> FBO_C (screen)

we would be trying to render the contents of both FBO_B and FBO_C in
the same flush, before the contents of Texture_A had made it to FBO_B.
So when FBO_C wants to use mipmaps of Texture_B they didn't exist yet
and appeared all black. And the blackness would remain for subsequent
frames as cogl has now decided the mipmaps of FBO_B are no longer
"dirty" and don't need refreshing:

  FBO_B (Texture_B) (mipmaps_dirty==FALSE but black) -> FBO_C (screen)

We must flush FBO_B before referencing Texture_B for use in rendering
FBO_C. This only happens when Texture_A changes (e.g. when the user
changes their background wallpaper) so there's no ongoing performance
penalty from this flush.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1347

(cherry picked from commit 3a474556b8)

cogl/cogl/cogl-texture-2d.c

@@ -402,6 +402,11 @@ _cogl_texture_2d_pre_paint (CoglTexture *tex, CoglTexturePrePaintFlags flags)
     {
       CoglContext *ctx = tex->context;
 
+      /* Since we are about to ask the GPU to generate mipmaps of tex, we
+       * better make sure tex is up-to-date.
+       */
+      _cogl_texture_flush_journal_rendering (tex);
+
       ctx->driver_vtable->texture_2d_generate_mipmap (tex_2d);
 
       tex_2d->mipmaps_dirty = FALSE;