Transition backend from vulkano to wgpu

[mitchmindtree]

There's been a lot of discussion around rendy and wgpu in the #nannou-dev channel of the nannou slack lately. The growing consensus is that wgpu, although slightly limited comparatively in some areas, may be the better fit for nannou in the long run. This issue is an alternative to #438, though adapted from it.

If you're interested in the discussion, I recommend reading through the discussions from the past month or two in #nannou-dev at our slack.

• Create a wgpu backend for conrod (PR here):
  • Geometry
  • Text
  • Images.
  • Allow for specifying size of glyph cache dimensions.
  • Ensure resizing behaves properly.
  • Add winit 0.21 conversion module to conrod_winit
  • Publish new version of conrod crates along with conrod_wgpu. PR here.
• Update nannou: see WIP - Transition graphics backend from vulkano to wgpu #452.
• Update the nannou guide:
  • Installation instructions (remove shaderc and macOS vulkan/moltenvk stuff). Add spirv-cross requirements (hopefully wgpu build scripts take care of most things?).
  • Update code snippets where necessary and run tests.
  • Publish the guide.

Edit: Remaining items have been moved to #452.

I currently have a local wgpu branch with the first few "update nannou" items done, but will wait until I get it compiling again before I open a branch here. Edit: See #452.