[gtk] (6 commits) ...Merge branch 'ngl-vertex-attributes' into 'master'

[Matthias Clasen <matthiasc src gnome org>]

Summary of changes:

  3252f1e... gsk: Give ngl its own shader sources (*)
  06d5c8e... ngl: Set color as vertex attribute (*)
  6a30c6b... ngl: Make the coloring shader more versatile (*)
  0ee6868... gtk-demo: Add a test for scrolling with Emoji (*)
  ebe30d8... ngl: Identify shaders (*)
  3e1524f... Merge branch 'ngl-vertex-attributes' into 'master'

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