Port swgl to no_std

[fschutt]

This PR makes webrender / swgl ready to be used on embedded hardware. The only thing that webrender / gleam uses from the entire standard library is CString / CStr conversions, which now have no_std replacement functions in gleam

The goal is to get webrender / azul running on a LinuxFb framebuffer or get it working with EGL for embedded Linux hardware.

Currently blocked on:

• gl-rs: make crate no_std compatible brendanzab/gl-rs#530
• gleam: make gleam no_std compatible gleam#221

TODO:

• Re-export cstring_from_str and cstr_from_ptr in gleam
• Make gleam::ffi::__gl_imports or expose the types
• Fix khronos API generation build error in gl-rs
• Version bump & dependency management

[bors-servo]

☔ The latest upstream changes (presumably #4808) made this pull request unmergeable. Please resolve the merge conflicts.

[mrobinson]

This repository is just a mirror ATM, so please contribute substantive changes to WebRender upstream at https://github.com/mozilla/gecko-dev/tree/24f9846b7d8ea0819edee7aafc51a39753f1619a/gfx/wr.