fix(CellArrayMapper): correct alpha channel handling to use texture alpha for transparency

Set computedColor.a to multiply mapperUBO.Opacity with the diffuse
texture’s alpha channel, ensuring proper transparency rendering.

fixes #3273

Author: daker

Sources/Rendering/WebGPU/CellArrayMapper/index.js

@@ -345,6 +345,8 @@ fn main(
 
   //VTK::Select::Impl
 
+  // Use texture alpha for transparency
+  computedColor.a = mapperUBO.Opacity * _diffuseMap.a;
   if (computedColor.a == 0.0) { discard; };
 
   //VTK::Position::Impl