551: Replace unsafe impl Pod with safe derive r=kvark a=rukai
bytemuck now includes derives that will implement the Pod trait, failing to compile if the struct cannot safely be a Pod.
Lets use it to remove most of the unsafe usage from the examples.
closes https://github.com/gfx-rs/wgpu-rs/issues/190
Co-authored-by: Rukai <rubickent@gmail.com>