iOS: use shaderc-rs for glsl to spirv compilation

[MichaelHills]

One of the steps to getting iOS working #87

I tested a few examples on mac sprite, sprite_sheet, and 3d_scene. On iOS I tested 3d_scene and a cut down version of spawner.

The motivating change for this was spawning glslValidator (or other processes) isn't going to work on iOS. Plus the bevy-glsl-to-spirv code said TODO use shaderc-rs.

[cart]

The biggest downsides here are that shaderc:

1. has a big effect on compile times (unless you bundle precompiled binaries)
2. is hard to build on windows (requires setting up the ninja build system and python). this makes onboarding difficult.
3. precompiled binaries require setting environment variables, which complicates builds and makes "first time usage" hard.

Medium term, Naga will handle this and wont suffer from these problems.

Short term, I would only want to merge this if we do one of the following:

1. Make shaderc-rs use precompiled binaries by default on all desktop platforms, with zero manual configuration required from users.
2. Use shaderc on IOS and glsl-to-spirv everywhere else (using cargo features).

[MichaelHills]

Makes sense, I prefer option 2 for now rather than making invasive changes to all platforms.

[MichaelHills]

I've updated to use cfg switches. However I noticed that @lachlansneff seems to have started on some naga support? https://github.com/bevyengine/bevy/commits/a47749c794761cc4c13879a1638a77cecaa06804/crates/bevy_render/src/shader/preprocessor.rs

I thought I'd see if I could get naga working for iOS instead, and when I noticed that preprocessor support was lacking I stumbled across gfx-rs/naga#132

So maybe hold off on shaderc-rs, Naga would be better and doesn't have any of the iOS cmake issues since its pure rust.

[lachlansneff]

Naga is quickly becoming mature enough to replace spirv-reflect in bevy.

[cart]

I'm sold on waiting for Naga. That seems like the right call. But if you hit a roadblock / want to move forward now, im also willing to merge "shaderc but only for iOS"

[MichaelHills]

Just rebased and re-pushed. I think we should just land this for now. We are getting some traction from multiple people on iOS support so let's land what we can to streamline the process.

There's still one outstanding issue with cmake-rs needing to be forked for now in order to compile shaderc-rs properly so this PR isn't enough on its own. It's just 1 less issue though. :) I'll keep an eye on Naga and try to port the iOS target to it when it's ready.

[cart]

Sounds good to me. I also just realized we could make this a temporary solution to #130 if we can somehow make shaderc optional for non-ios platforms and required for ios.

But no sense in blocking on that. This is good to go.