Example: Add webgpu_postprocessing_masking

[sunag]

Related issue: Closes #29008

Description

I fork the example of @cmhhelgeson to create the simplified version.

We can have this effect with:

const base = pass( baseScene, camera );
const sceneMask1 = pass( maskScene1, camera ).a;
const sceneMask2 = pass( maskScene2, camera ).a;

let compose = base;
compose = sceneMask1.mix( compose, texture( texture1 ) );
compose = sceneMask2.mix( compose, texture( texture2 ) );

postProcessing.outputNode = compose;