gdk: Add with_* constructors for RGBA

gdk4/src/rgba.rs

@@ -79,6 +79,40 @@ impl RGBA {
         }
     }
 
+    #[inline]
+    pub const fn with_red(self, red: f32) -> Self {
+        Self {
+            inner: ffi::GdkRGBA { red, ..self.inner },
+        }
+    }
+
+    #[inline]
+    pub const fn with_green(self, green: f32) -> Self {
+        Self {
+            inner: ffi::GdkRGBA {
+                green,
+                ..self.inner
+            },
+        }
+    }
+
+    #[inline]
+    pub const fn with_blue(self, blue: f32) -> Self {
+        Self {
+            inner: ffi::GdkRGBA { blue, ..self.inner },
+        }
+    }
+
+    #[inline]
+    pub const fn with_alpha(self, alpha: f32) -> Self {
+        Self {
+            inner: ffi::GdkRGBA {
+                alpha,
+                ..self.inner
+            },
+        }
+    }
+
     // rustdoc-stripper-ignore-next
     /// Creates a new builder-pattern struct instance to construct [`RGBA`] objects.
     ///