message: Load only gl 3.3 functions
  This commit makes 'gl::load_with' only load
  symbols from the minimal OpenGL (3.3) version alacritty
  aims to support. Doing so reduces the size of gl_bindings.rs
  from 21K LoC to 11K LoC, and also the actual
  loading by up to ~2x, thugh the loading is usually sub millisecond
  anyway.
diff: +use std::collections::HashSet;
  use std::env;
  use std::fs::File;
  use std::path::Path;
  @@ -11,7 +12,7 @@ fn main() {
  let dest = env::var("OUT_DIR").unwrap();
  let mut file = File::create(&Path::new(&dest).join("gl_bindings.rs")).unwrap();
  - Registry::new(Api::Gl, (4, 5), Profile::Core, Fallbacks::All, ["GL_ARB_blend_func_extended"])
  + Registry::new(Api::Gl, (3, 3), Profile::Core, Fallbacks::All, ["GL_ARB_blend_func_extended"])
  .write_bindings(GlobalGenerator, &mut file)
  .unwrap();
  