Add support for iOS

[hannobraun]

Broad platform support (at least all major desktop and mobile platforms, as well as the web) is a goal of Fornjot. This issue tracks support for iOS. iOS is supported by winit and wgpu, so it should be possible to port Fornjot without too much extra effort.

I don't have a vision for mobile support in general, except that it should be possible to show people your models on your phone or tablet. I also don't know if it would be better to add native support, or somehow piggy-back on top of web support (#73). In any case, #71 might be a blocker.

For a start, it would be great to be able to run Fornjot on iOS within a development environment, as well as extending the CI build to make sure that support doesn't deteriorate. Once that is achieved, this issue can be closed and more specific follow-up issues can be opened.

I have zero experience with iOS development, don't own any Apple devices, and personally don't see this as a priority at the moment. Adding https://github.com/hannobraun/Fornjot/labels/help%20wanted, as any iOS support will be dependent on contributors driving the effort, at least for now.

Also see #152, which tracks support for Android.

[hannobraun]

I've looked into this (and Android support; see #1278). I tried to build for aarch64-apple-ios in the same way I'm already building for the Android target in #1278, but I'm getting this error:

error: failed to run custom build command for `objc_exception v0.1.2`

Caused by:
  process didn't exit successfully: `/home/hanno/Projects/main/fornjot/target/debug/build/objc_exception-a6502a644475d577/build-script-build` (exit status: 1)
  --- stdout
  TARGET = Some("aarch64-apple-ios")
  OPT_LEVEL = Some("0")
  HOST = Some("x86_64-unknown-linux-gnu")
  CC_aarch64-apple-ios = None
  CC_aarch64_apple_ios = None
  TARGET_CC = None
  CC = None
  CFLAGS_aarch64-apple-ios = None
  CFLAGS_aarch64_apple_ios = None
  TARGET_CFLAGS = None
  CFLAGS = None
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some("true")
  Detecting iOS SDK path for iphoneos
  running: "xcrun" "--show-sdk-path" "--sdk" "iphoneos"

  --- stderr


  error occurred: Failed to find tool. Is `xcrun` installed?

I don't know what xcrun is, but I suspect I need macOS to run it. It might be, that we need to update the cross-compiling infrastructure such that it only happens on GitHub Actions, and uses the appropriate runner for each target. Doable, but it's not something I personally want to attack right this moment. (Help welcome though, if you're interested!)