wgpu-rs render backend

[Dinnerbone]

This will solve #14, at least for desktop. It's structured similarly to #520 with the renderer pulled out into its own subcrate.

This provides a wgpu-rs backend for rendering, that is completely agnostic to web or desktop. It could in theory be used today on web, provided your browser has support for it and someone hooks the thing up to web...

The API is extremely similar to Vulkan and isn't very suited to the way that we render today. After we pull this in, I'd like to change our renderer API to work on a frame object so that we only need to do one renderpass per frame and not unlimited per frame. I don't want this PR to break API of other renderers though (especially as #520 is still WIP).

This is still a work in progress, opening a draft PR so it's visible for anyone who wants to see, and to track remaining progress.

Remaining tasks:

• Bitmap object rendering (outside of shape rendering)
• Mask support
• Letterbox drawing
• Add a feature flag for labeling all buffers/etc, for debugging & performance analysis
• Split off into smaller files
• Optimization pass

[Dinnerbone]

I've done a basic look at optimising but unfortunately I think the render API needs to change to get any significant wins. I'll look into that after this PR. I'll count this as ready for review.

[Herschel]

Looks great, thanks! I still think there is some benefit to putting some common things in common_tess, but this can wait until after other renderer improvements. It might also be useful to allow switching between the vulkan/metal/dx12 backends of wgpu (via feature/command line).