[mutter] (6 commits) Non-fast-forward update to branch gbsneto/background-content

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

The branch 'gbsneto/background-content' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  30251f8... Introduce MetaBackgroundContent
  162485a... background-actor: Use MetaBackgroundContent
  7a7c99c... background-content: Simplify method call
  f4f6822... background-content: Render background slices relative to ac

Commits added to the branch:

  74c0d91... stack-tracker: Fix coding style of meta_stack_op_apply() (*)
  53f61f3... stack-tracker: Don't log warnings on race conditions (*)
  a3ec37b... Introduce MetaBackgroundContent
  6f65c4d... background-actor: Use MetaBackgroundContent
  f43c3f5... background-content: Simplify method call
  d4accda... background-content: Render background slices relative to ac

(*) This commit already existed in another branch; no separate mail sent