backdrop-filter: should create stacking context in servo.

Author: JoshuaBrest

style/properties/longhands/effects.mako.rs

@@ -59,7 +59,8 @@ ${helpers.predefined_type(
     "backdrop-filter",
     "Filter",
     None,
-    engines="gecko",
+    engines="gecko servo",
+    servo_pref="layout.backdrop-filter.creates-stacking-context",
     vector=True,
     simple_vector_bindings=True,
     gecko_ffi_name="mBackdropFilters",