anti-lag: Do not enable layer by default.

There are too many reports of instability in the wild,
even in scenarios where anti-lag isn't really being actively used,
making it questionable to load the layer by default.

By using enable_environment, a certain environment variable
must be set for loader to consider loading the layer.
This could be removed once the layer stabilizes.

Cc: mesa-stable

Signed-off-by: Hans-Kristian Arntzen <post@arntzen-software.no>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37509>

src/vulkan/anti-lag-layer/VkLayer_MESA_anti_lag.json.in

@@ -21,6 +21,9 @@
       ],
       "disable_environment": {
          "DISABLE_LAYER_MESA_ANTI_LAG": "1"
+      },
+      "enable_environment": {
+         "ENABLE_LAYER_MESA_ANTI_LAG": "1"
       }
    }
 }