Fix gpu picking instances when instances are disposed (#16528)

From:
https://forum.babylonjs.com/t/removing-thin-instances-vs-gpupicker/57989/5

Disposing an instance fouls up the gpu picking result. This is due to
changes in the order of instances in `mesh.instances` . Fixed correct
mapping of color id of instances to pickable instances by checking
against `_pickableMeshes` instead.

Author: lockphase

packages/dev/core/src/Collisions/gpuPicker.ts

@@ -11,6 +11,7 @@ import type { IColor3Like, IVector2Like } from "core/Maths/math.like";
 import type { AbstractMesh } from "core/Meshes/abstractMesh";
 import { VertexBuffer } from "core/Meshes/buffer";
 import type { Mesh } from "core/Meshes/mesh";
+import type { InstancedMesh } from "core/Meshes/instancedMesh";
 import { Logger } from "core/Misc/logger";
 import type { Scene } from "core/scene";
 import type { Nullable } from "core/types";
@@ -317,13 +318,14 @@ export class GPUPicker {
                 id++;
 
                 if (mesh.hasInstances) {
-                    const instances = (mesh as Mesh).instances;
-                    const colorData = this._generateColorData(instances.length, id, index, GPUPicker._TempColor.r, GPUPicker._TempColor.g, GPUPicker._TempColor.b, (i, id) => {
-                        const instance = instances[i];
+                    const numInstances = (mesh as Mesh).instances.filter((instance) => this._pickableMeshes.indexOf(instance) !== -1).length;
+                    const allInstancesForPick = this._pickableMeshes.filter((m) => m.isAnInstance && (m as InstancedMesh).sourceMesh === mesh);
+                    const colorData = this._generateColorData(numInstances, id, index, GPUPicker._TempColor.r, GPUPicker._TempColor.g, GPUPicker._TempColor.b, (i, id) => {
+                        const instance = allInstancesForPick[i];
                         this._idMap[id] = this._pickableMeshes.indexOf(instance);
                     });
 
-                    id += instances.length;
+                    id += numInstances;
                     const engine = mesh.getEngine();
 
                     const buffer = new VertexBuffer(engine, colorData, this._attributeName, false, false, 4, true);