[gtk] (18 commits) Non-fast-forward update to branch css-style-split

[Matthias Clasen <matthiasc src gnome org>]

The branch 'css-style-split' 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:

  acd0453... css: Split style into groups
  2f2e535... Add accounting for styles
  891d7ba... Add some checking for the group splitup

Commits added to the branch:

  4287f0d... window: Replace GtkStyleContext use to get surface transfor (*)
  1a56a10... cssimage: Add gtk_css_image_to_string() (*)
  f62535a... gl renderer: Avoid some work when rendering border nodes (*)
  69fe705... GtkCssImageUrl: Implement is_computed() (*)
  36e3f4d... cssprovider: Parse selectors without a GList (*)
  355a417... cssparser: make a few local variables const (*)
  7ccd659... Remove gtkutils{.c,private.h} (*)
  942a932... render: Remove gtk_render_background_get_clip (*)
  77e0d36... Add pure and const annotations to various functions (*)
  2e25698... renderbackground: Avoid unnecessary work in snapshot_backgr (*)
  30b37dd... widget: Restructure pick() a bit (*)
  352c9ba... dimensionvalue: Add gtk_css_{dimension,corner}_value_is_zer (*)
  c7ea83c... cssboxes: Try to avoid doing work for rectangular boxes (*)
  a16e1cb... cssdimensionvalue: Remove an outdated comment (*)
  05b9a99... gsk: Remove some unused defines (*)
  1a4d9ca... css: Split style into groups
  09c8b74... Add accounting for styles
  4aae706... Add some checking for the group splitup

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