ShadowGenerator: optimize renderList parsing in Parse (#17034)

The `ShadowGenerator.Parse` method exhibited a performance bottleneck
due to its inefficient handling of the `renderList`. Previously, it
iterated through the `parsedShadowGenerator.renderList` and for each
mesh ID, it called `scene.getMeshesById`. This resulted in an O(m*n)
time complexity, where 'n' is the total number of meshes in the scene
and 'm' is the number of meshes in the `renderList`. Profiling indicated
that `getMeshesById` was a significant performance drain.

This commit refactors the parsing logic to improve efficiency. Instead
of repeatedly querying meshes by ID, it now creates a `Set` of mesh IDs
from the `renderList` (O(m) time and space). It then iterates through
all meshes in the scene once (O(n) time) and checks for membership in
the `Set` using an O(1) average time complexity lookup. Any mesh whose
ID is found in the set is added to the `shadowMap.renderList`.

This change reduces the overall time complexity of parsing the
`renderList` to O(n + m), which is a substantial performance
improvement, especially for scenes with a large number of meshes or a
large `renderList`.

Forum post:
<https://forum.babylonjs.com/t/optimize-performance-of-shadowgenerator-parse/60086>

---------

Co-authored-by: sebavan <[email protected]>

Author: kzhsw

packages/dev/core/src/Lights/Shadows/shadowGenerator.ts

@@ -2155,16 +2155,17 @@ export class ShadowGenerator implements IShadowGenerator {
         const shadowGenerator = constr ? constr(parsedShadowGenerator.mapSize, light, camera) : new ShadowGenerator(parsedShadowGenerator.mapSize, light, undefined, camera);
         const shadowMap = shadowGenerator.getShadowMap();
 
-        for (let meshIndex = 0; meshIndex < parsedShadowGenerator.renderList.length; meshIndex++) {
-            const meshes = scene.getMeshesById(parsedShadowGenerator.renderList[meshIndex]);
+        if (parsedShadowGenerator.renderList.length && shadowMap) {
+            const renderSet = new Set<string>(parsedShadowGenerator.renderList);
+            let renderList = shadowMap.renderList;
+            if (!renderList) {
+                renderList = shadowMap.renderList = [];
+            }
+            const meshes = scene.meshes;
             for (const mesh of meshes) {
-                if (!shadowMap) {
-                    continue;
-                }
-                if (!shadowMap.renderList) {
-                    shadowMap.renderList = [];
+                if (renderSet.has(mesh.id)) {
+                    renderList.push(mesh);
                 }
-                shadowMap.renderList.push(mesh);
             }
         }