SepiaNode: Support transparent background (#29130)

src/nodes/display/SepiaNode.js

@@ -1,4 +1,4 @@
-import { addNodeElement, Fn, vec3 } from '../shadernode/ShaderNode.js';
+import { addNodeElement, Fn, vec3, vec4 } from '../shadernode/ShaderNode.js';
 import { dot } from '../math/MathNode.js';
 
 export const sepia = /*@__PURE__*/ Fn( ( [ color ] ) => {
@@ -7,10 +7,11 @@ export const sepia = /*@__PURE__*/ Fn( ( [ color ] ) => {
 
 	// https://github.com/evanw/glfx.js/blob/master/src/filters/adjust/sepia.js
 
-	return vec3(
+	return vec4(
 		dot( c, vec3( 0.393, 0.769, 0.189 ) ),
 		dot( c, vec3( 0.349, 0.686, 0.168 ) ),
-		dot( c, vec3( 0.272, 0.534, 0.131 ) )
+		dot( c, vec3( 0.272, 0.534, 0.131 ) ),
+		color.a
 	);
 
 } );