Transition graphics backend from vulkano to rendy

[mitchmindtree]

This issue is for tracking progress on the transition from vulkano to rendy for nannou's graphics backend.

The following is a roughly ordered overview of the steps involved:

• Create a rendy backend for conrod (WIP branch here):
  • Geometry
  • Text
  • Images (Almost done - need to work out how to switch between images for a single sampler).
  • Perhaps change ConrodAux to a trait to allow for adding ConrodPipeline nodes within Graphs implemented for different "auxiliary" types.
  • Allow for specifying size of glyph cache dimensions.
  • Remove GLSL shader compilation feature in favour of including pre-compiled SPIR-V shader.
  • Ensure resizing behaves properly.
  • Remove shader-reflection feature in favour of manually specifying layout if possible.
  • Move winit conversion updates into conrod_winit
  • Remove image crate dependency in favour of using in example directly.
  • PR + request review from rendy devs.
  • Publish new version of conrod along with conrod_rendy
• Update nannou:
  • Create a nannou/rendy branch that compiles with all graphics related stuff commented out / removed.
  • Use winit via rendy rather than a separate import.
  • Update app and window modules to manage windows and swapchains via rendy rather than gfx.
  • Re-write the run_loop for changes to winit::EventLoop. We should no longer use poll_events, but instead use EventsCleared along with some timer. This will likely require a full re-write. May be easiest to introduce one LoopMode at a time.
  • Update the frame.rs module. We may want to re-think the Frame de# terms of rendy's Graph?
  • Update ui.rs module to use conrod_rendy rather than conrod_vulkano.
  • Update draw.rs module. Update draw.rs module #440.
  • Change vk examples directory to rendy and update all examples.
  • Remove vk.rs module.
  • Create screenshot.rs example.
  • Ensure CHANGELOG is up-to-date.
  • Publish new version of nannou.
  • Update the guide installation instructions (remove shaderc and macOS vulkan stuff).

To read more about the reasoning behind the change, see #374 and its related issues.

cc @freesig, @DavidPartouche, @JoshuaBatty