[gtk] (10 commits) ...Merge branch 'wip/otte/memoryformat' into 'master'

[Benjamin Otte <otte src gnome org>]

Summary of changes:

  b65b646... texture: Refactor downloading (*)
  7b41738... png: Do loader conversions in GDK (*)
  1e7fb52... Add memory formats used by libpng (*)
  c2368cc... png: Refactor png saving (*)
  31e9072... gl: Fix downloading textures *again* (*)
  5199beb... gl: Move texture uploading to the renderer (*)
  52e88ed... gltexture: Determine format in current thread (*)
  ddc4a40... gl: Refactor texture uploading (*)
  9a2e9c5... gl: Use gdk_memory_texture_new_from_texture() (*)
  a7ef4c7... Merge branch 'wip/otte/memoryformat' into 'master'

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