gitlab-ci: add a new GIMP_CI_RASTER_ICONS to test raster icon build.

Since this is probably much less tested than the default vector icon
case, any bug there might go unseen for some time. So let's add a new
pipeline to check this. I make so this pipeline is not actually run on
every commit (i.e. manually only), then I'll just add a scheduled
pipeline to check the non-vector icon case at regular intervals.

This pipeline also test the icon-list.mk files are not outdated.

.gitlab-ci.yml

@@ -24,6 +24,7 @@
 # - GIMP_CI_SOURCES: trigger the autotools/gcc build and the source tarball job.
 # - GIMP_CI_CPPCHECK: trigger cppcheck static analysis.
 # - GIMP_CI_FLATPAK: trigger the nightly flatpak build and publishing.
+# - GIMP_CI_RASTER_ICONS: trigger the autotools and meson builds without vector icons.
 
 include: 'https://gitlab.gnome.org/GNOME/citemplates/raw/master/flatpak/flatpak_ci_initiative.yml'
 
@@ -57,6 +58,7 @@ build-image:
     - if: '$GIMP_CI_SOURCES != null'
     - if: '$GIMP_CI_MESON_GCC != null'
     - if: '$GIMP_CI_MESON_CLANG != null'
+    - if: '$GIMP_CI_RASTER_ICONS != null'
   stage: prepare
   variables:
     GIT_STRATEGY: none
@@ -138,6 +140,7 @@ deps-debian:
     - if: '$GIMP_CI_SOURCES != null'
     - if: '$GIMP_CI_MESON_GCC != null'
     - if: '$GIMP_CI_MESON_CLANG != null'
+    - if: '$GIMP_CI_RASTER_ICONS != null'
   stage: dependencies
   image: $CI_REGISTRY_IMAGE:build-debian-latest
   cache:
@@ -249,6 +252,38 @@ gimp-clang-debian:
     - ninja -C _build
     - ninja -C _build test
 
+gimp-meson-raster-icons:
+  rules:
+    # Custom builds only (web GUI, API or schedules).
+    - if: '$GIMP_CI_RASTER_ICONS != null'
+  extends: .gimp-debian-base
+  script:
+    - meson _build
+       --prefix="${INSTALL_PREFIX}"
+       -Dvector-icons=false
+    - ninja -C _build
+    - ninja -C _build test
+
+gimp-autotools-raster-icons:
+  rules:
+    # Custom builds only (web GUI, API or schedules).
+    - if: '$GIMP_CI_RASTER_ICONS != null'
+  extends: .gimp-debian-base
+  script:
+    - tools/generate-icon-makefiles.py
+    - diff="$(git diff)"
+    - if [ -n "$diff" ]; then
+        echo "Outdated icon-list.mk - please run tools/generate-icon-makefiles.py";
+        echo "$diff";
+        exit 1;
+      fi
+    - mkdir _build
+    - cd _build
+    - ../autogen.sh
+        --prefix="${INSTALL_PREFIX}"
+        --disable-vector-icons
+    - make -j "$(nproc)"
+
 ## WINDOWS 64-bit CI (native MSYS2) ##
 
 deps-win64-native: