Perf: Ensure transparent terrain fragments are discarded

Setting the alpha to zero has the same net effect, but it wastes GPU compute time as the rest of the shader program might still be executed - for no good reason, given that the fragment won't be visible anyway.

Core/NativeClient/WebGPU/Shaders/TerrainGeometryShader.wgsl

@@ -149,7 +149,7 @@ fn fs_main(in: VertexOutput) -> @location(0) vec4f {
 	let ambientColor = uPerSceneData.ambientLight.rgb;
 
 	if (isTransparentBackgroundPixel(diffuseTextureColor)) {
-		diffuseTextureColor.a = 0.0;
+		discard;
 	}
 
 	let lightmapTexCoords = in.lightmapTextureCoords;