GPUPicker: Fix picking ID precision issues (#17404)

Resolves the issue:

https://forum.babylonjs.com/t/missing-doc-for-gpupicker-in-v8-31-3/61295/5

What I found while examining this issue is that

when outputting values received as uniform variables in the shader to
the screen,
the uniform values were displayed completely without error,

but values received as attributes exhibited errors.

I don't know the exact reason for this, as it is an internal WebGL
implementation detail.
Through experimentation, I found a safe `MaxPickingId` value usable
across WebGL, WebGL2, and WebGPU.
I confirmed that adding 0.5 at the point of casting from int resolves
this precision issue.

Author: noname0310

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

@@ -219,8 +219,6 @@ export class GPUPicker {
             return;
         }
 
-        this._pickableMeshes = list as Array<AbstractMesh>;
-
         // Prepare target
         const scene = ("mesh" in list[0] ? list[0].mesh : list[0]).getScene();
         if (!this._cachedScene || this._cachedScene !== scene) {

packages/dev/core/src/Shaders/picking.fragment.fx

@@ -1,6 +1,6 @@
 
 #if defined(INSTANCES)
-varying float vMeshID;
+flat varying float vMeshID;
 #else
 uniform float meshID;
 #endif

packages/dev/core/src/Shaders/picking.vertex.fx

@@ -16,7 +16,7 @@ uniform mat4 viewProjection;
 
 // Output
 #if defined(INSTANCES)
-varying float vMeshID;
+flat varying float vMeshID;
 #endif
 
 void main(void) {

packages/dev/core/src/ShadersWGSL/picking.fragment.fx

@@ -1,6 +1,6 @@
 
 #if defined(INSTANCES)
-varying vMeshID: f32;
+flat varying vMeshID: f32;
 #else
 uniform meshID: f32;
 #endif

packages/dev/core/src/ShadersWGSL/picking.vertex.fx

@@ -16,7 +16,7 @@ uniform viewProjection: mat4x4f;
 
 // Output
 #if defined(INSTANCES)
-varying vMeshID: f32;
+flat varying vMeshID: f32;
 #endif
 
 @vertex