345: Rustify the example's use of Extensions and SamplerDescriptor r=kvark a=cwfitzgerald
Implements https://github.com/gfx-rs/wgpu/pull/696.
This didn't change any content from #338, just interface, but I tested a couple examples and it seems to work okay.
Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>