[gdk-pixbuf] (16 commits) ...Merge branch 'ebassi/gi-docgen' into 'master'

[Emmanuele Bassi <ebassi src gnome org>]

Summary of changes:

  0a670de... docs: Fix the deprecation annotation for AlphaMode (*)
  073a947... Fix inclusion guards for gdk-pixbuf-features.h (*)
  0c545e3... Port gdk-pixbuf to gi-docgen (*)
  0ad5532... docs: Port to the gi-docgen syntax and style (*)
  9de2b14... docs: Generate the GdkPixdata API reference (*)
  baf188d... ci: Update the Docker image (*)
  5030bb1... ci: Update reference and dist jobs (*)
  63e3081... docs: Clean up GdkPixbuf's core API (*)
  f2a1123... docs: Update gdk-pixbuf-io style (*)
  27f5e96... docs: Update pixdata style (*)
  1dd020c... docs: Update pixbuf-data style (*)
  8faf466... docs: Update pixbuf-util style (*)
  3b49374... docs: Update pixbuf-scale style (*)
  d2cb184... docs: Update pixbuf-animation style (*)
  f75f9bf... docs: Update pixbuf-loader style (*)
  5e1ecb4... Merge branch 'ebassi/gi-docgen' into 'master'

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