Improve execution of examples in CI (#9331)

# Objective

- Some examples crash in CI because of needing too many resources for
the windows runner
- Some examples have random results making it hard to compare
screenshots

## Solution

- `bloom_3d`: reduce the number of spheres
- `pbr`:  use simpler spheres and reuse the mesh
- `tonemapping`: use simpler spheres and reuse the mesh
- `shadow_biases`: reduce the number of spheres
- `spotlight`: use a seeded rng, move more cubes in view while reducing
the total number of cubes, and reuse meshes and materials
- `external_source_external_thread`, `iter_combinations`,
`parallel_query`: use a seeded rng

Examples of errors encountered:
```
Caused by:
    In Device::create_bind_group
      note: label = `bloom_upsampling_bind_group`
    Not enough memory left
```

```
Caused by:
    In Queue::write_buffer
    Parent device is lost
```
```
ERROR wgpu_core::device::life: Mapping failed Device(Lost)
```