Improve execution of examples in CI

[mockersf]

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)

[alice-i-cecile]

These all seem like reasonable, tiny changes in service of an important goal.

[alice-i-cecile]

Merging as trivial.