app: Fuzzy Select tool multi-layer aware.

When several layers are selected, and using the Fuzzy select tool
without sample merged (with sample merged, nothing changes, it just uses
the whole image), the selection will be based off a composited render of
the image with only selected layers.

This is equivalent to only making selected layers visible, running fuzzy
select with sample merged, then making other layers visible again.
12 jobs for master in 38 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #836663
build-image

00:03:36

passed #836665
build-image-win32

00:03:04

passed #836664
build-image-win64

00:03:29

 
  Dependencies
passed #836666
deps-debian/testing

00:02:25

passed #836668
deps-win32

00:13:37

passed #836667
deps-win64

00:09:26

 
  Gimp
passed #836669
gimp-debian/testing-autotools

00:11:20

passed #836671
gimp-debian/testing-clang

00:23:12

passed #836670
gimp-debian/testing-meson

00:05:16

passed #836673
gimp-win32

00:08:38

passed #836672
gimp-win64

00:25:04

 
  Analysis
passed #836674
cppcheck

00:02:08