[libadwaita/wip/exalm/split-button] stylesheet: Fix AdwSplitButton disabled state

[Alexander Mikhaylenko <alexm src gnome org>]

commit d0a7643aef2a56d707decc27641066647470c800
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Mon Aug 8 15:05:38 2022 +0400

    stylesheet: Fix AdwSplitButton disabled state

 src/stylesheet/widgets/_buttons.scss | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/src/stylesheet/widgets/_buttons.scss b/src/stylesheet/widgets/_buttons.scss
index 7e7b70ff..5f1ee606 100644
--- a/src/stylesheet/widgets/_buttons.scss
+++ b/src/stylesheet/widgets/_buttons.scss
@@ -478,6 +478,14 @@ splitbutton {
     }
   }
 
+  &:disabled {
+    filter: opacity($disabled_opacity);
+
+    > button, > menubutton > button {
+      filter: none;
+    }
+  }
+
   // Reimplementing linked so we don't blow up css
   > button:dir(ltr),
   > menubutton > button:dir(rtl) {
@@ -513,6 +521,14 @@ splitbutton {
       }
     }
 
+    &:disabled {
+      filter: opacity($strong_disabled_opacity);
+
+      > button:disabled, > menubutton > button:disabled {
+        filter: none;
+      }
+    }
+
     &:focus-within:focus-visible > separator {
       background: none;
     }