Update WGPU to v0.13

[dhardy]

Replaces #331.

One shader was translated to WGSL as a proof of concept. GLSL shaders will remain to ease any future OpenGL implementation; in general there doesn't seem to be motivation to switch the shader language.

[dhardy]

There is a motivation to switch to WGSL: no need for a GLSL compiler and the option to embed WGSL in Rust. I translated Mandlebrot's shaders in this branch.

But, I feel, there are significant drawbacks:

• GLSL code is much cleaner
• The WGSL spec is incomplete (e.g. no mention of f64 support yet, which does work, though literals appear to be missing)
• WGSL validation for embedded shaders only happens at run-time (though naga can be run directly for fast validation)