Draft: Introduce the GL integration crate

Introduce the GL integration crate: a crate that retrieves gst_gl::GLContext and gst_gl::GLDisplay from "common" sources in the Rust ecosystem: a RawWindowHandle, and as bonus also from glutin::WindowedContext.

This substitutes the matching logic that was originally in the glupload example to be expandable and reusable by other crates, similar to how the ash-window crate exists for ash (turns RawWindowHandles into Vulkan Surface).

Since a GLDisplay acquired from a RawWindowHandle is only half the info we need and has an incomplete implementation currently (no support for Android or Windows), a second function retrieves GLContext from glutin together with display fallbacks for EGL-based windows.

Most interesting is probably to find a public crate or two using these bindings, and see if we can migrate them over to this crate as a final review of the API and versatility. At the same time they might support more platforms too. So far crates.io shows only a single crate, supporting Wgl and Glx: https://github.com/marek-g/rust-media/blob/master/media_gstreamer/src/pipeline_factory.rs.

Remaining tasks

• TODO comments in code;
• Deal with Wgl handles to support Windows;
• XCB constructor in gstreamer-base (backported, or with a fallback written in pure Rust);
• Returning Result/Option instead of panicking;
• Move code out of cfg_if to:
  • Have rustfmt work on the contents;
  • Get rid of some unexpected clippy warnings related to use items after statements (despite supposedly sitting at the top of a #[cfg] block - and #[allow] not being allowed inside the cfg_if block either);
  • Reduce indentation and make it more readable overall;
• Documentation, readme, etc.