Shadow artifacts with WebGPURenderer VSMShadowMap

[trusktr]

A solution for the shadow outline artifact is to patch ShadowNode.prototype.setupShadowFilter to fade out shadows around the edges of the shadow map area, like so:

			const originalSetupShadowFilter = THREE.ShadowNode.prototype.setupShadowFilter;

			THREE.ShadowNode.prototype.setupShadowFilter = function ( builder, inputs ) {

				// portion of shadow map UV space along edges to fade
				const fadePortion = 0.05;

				const base = originalSetupShadowFilter.call( this, builder, inputs );
				const uv = inputs.shadowCoord.xy;

				// Distance to nearest edge of the shadow UV box
				const edgeDist = min( min( uv.x, uv.x.oneMinus() ), min( uv.y, uv.y.oneMinus() ) );

				// Fade: 1 at the very edge, 0 inside (only if in a shadow)
				const fade = sub( 1, smoothstep( 0, fadePortion, edgeDist ) );
				return mix( base, 1, fade );

			};