build: disable Vulkan by default

We don't want people to build Vulkan support when they just want to get
GTK built.

This is in particular true for GTK as a CI subproject or for people
using jhbuild.

Worse, just having Vulkan support compiled in tends to cause crashes
in the Inspector, even if you are not using it.

.gitlab-ci/test-docker.sh

@@ -15,7 +15,7 @@ meson \
         -Dx11-backend=true \
         -Dwayland-backend=true \
         -Dbroadway-backend=true \
-        -Dvulkan=yes \
+        -Dvulkan=enabled \
         -Dprofiler=true \
         --werror \
         ${EXTRA_MESON_FLAGS:-} \

meson_options.txt

@@ -50,8 +50,8 @@ option('print-cups',
 
 option('vulkan',
        type: 'feature',
-       value: 'auto',
-       description : 'Enable support for the Vulkan graphics API')
+       value: 'disabled',
+       description : 'Enable support for the experimental Vulkan graphics API')
 
 option('xinerama',
        type: 'feature',