blur-effect: Port to paint nodes

Port the blur effect to the new ClutterEffect.paint_node() vfunc.
Update the function names to match what they do, e.g. "apply_blur()"
now creates the blur subtree and thus was appropriately renamed to
"create_blur_nodes()".

There are 3 subtrees that can be generated by the blur effect:

 1. Actor mode (full subtree; no cache)

      Root
       |----------------------------
       |                            |
    Layer (brightness)           Pipeline
       |                      (final result)
    Layer (horizontal blur)
       |
    Layer (vertical blur)
       |
    Layer (actor)
       |
    Transform (downscale)
       |
     Actor

 2. Actor mode (partial subtree; cached contents)

      Root
       |
     Pipeline
  (final result)

 3. Background mode

      Root
       |-------------------------------------------
       |                            |              |
    Layer (brightness)           Pipeline        Actor
       |                      (final result)
    Layer (horizontal blur)
       |
    Layer (vertical blur)
       |
    Layer (background)
       |
      Blit

!1339
9 jobs for !1339 with gbsneto/effects-paint-nodes in 4 minutes and 58 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #788493
check_commit_log

00:00:22

passed #788495
eslint

00:01:09

passed #788494
js_check

00:01:22

passed #788497
no_template_check

00:00:32

passed #788496
potfile_check

00:00:37

 
  Build
passed #788498
build

00:02:26

passed #788499
flatpak
flatpak

00:02:23

 
  Test
passed #788500
test

00:01:01

passed #788501
test-pot

00:01:09