Port shaders to WGSL

[parasyte]

This is a followup to #134 (comment)

I have an experimental Work In Progress that uses a slightly older version of naga to compile the WGSL. It needs to be updated with the latest changes contributed by the community, which includes wgpu 0.7. I haven't published a branch for it, yet. It needs some cleanup and updates.

The experimental WIP uses wgpu 0.6, so I had to compile WGSL to SPIR-V before passing the shader to wgpu. 0.7 is able to accept WGSL directly without the intermediate compilation step. There may also have been some syntax changes to WGSL since I last worked on it.

Finally, only the shader for the scaling renderer has been ported. The custom shader example still needs work.

[parasyte]

I'm going to want this before starting on #170. It's going to need an example with a custom shader and I would rather not write it twice in two different languages. I already have to rewrite the shaders for the scaling renderer and the custom shader example!

And recent naga releases should have fixed some of the outstanding issues I ran into last time. Now is a good time to pick up this work again.