Add WebGPU support to the Picker

[mvaligursky]

Picker renders mesh indices to the texture during preparation. When picking, a rectangle of this rendered texture is copied back to the CPU, and indices are used to identify mesh instances.

This is currently a synchronous process. The API needs to be changed to be async, allowing async texture readback on WebGPU. Related task to implement this part: https://github.com/playcanvas/engine/issues/2974