[Tracking Issue] Updating wgpu to 0.7

[mitchmindtree]

Here is the CHANGELOG entry for wgpu-rs. Some exciting stuff in there like early WGSL support and better error messages which might help to solve issues like #719.

Updating conrod_wgpu

The first step is to update conrod_wgpu to also target version 0.7. I've made a couple of short, failed attempts at this so far.

From memory, after running cargo update, one of rusttype's dependencies had broken for some reason or another, so I tried updating rusttype itself. Unfortunately one of the APIs that rusttype provided was removed in the latest version as it was assumed no-one was using it 😱

This lead me to refactor conrod_core's text rendering a little to avoid needing the API, which turned out for the better and should result in slightly better performance. However, along with the removal of the API, another issue was introduced in rusttype that made it impossible to console the lifetime requirements between PositionedGlyphs yielded by the Font::layout and the GPU Cache type.

This has since been fixed in master, however it turns out that rusttype has been looking for a maintainer for the last year and it is unlikely these changes will be published until someone steps up. Unfortunately, I already have crates coming out my ears and don't think it would be healthy for me to take on another. If anyone reading this would like to take it on please consider commenting at the repo!

The previous rusttype maintainer alludes to a successor crate called ab_glyph which boasts slightly better performance. Both conrod and nannou lean on rusttype quite heavily for text layout, and I must admit that I'm not particularly excited about the idea of rewriting of conrod's text layout and rendering to use ab_glyph just to update from wgpu 0.6 to 0.7, particularly as I would like to eventually replace conrod with nannou_ui and could be spending this time doing so.

I'm unsure of how to progress from here just yet. More than likely I'll end up raising my hand to help maintain rusttype temporarily... at least until we no longer depend on conrod for UI support.

[mitchmindtree]

Ahh, it seems the issue with the rusttype 0.8 dependency has since been resolved - I've landed those rusttype changes now and am currently looking into @yutannihilation's PR here PistonDevelopers/conrod#1410.