rust-opengles-angle: added ANGLE and EGL binding.

- added ANGLE into angle/.
- applied a local patch (angle/angle-rust-opengles-angle-local.patch) to
  ANGLE for the compilation with MinGW.
- added a custom Makefile (angle/src/Makefile) for ANGLE.
- added an EGL binding (egl.rs.in). currently only built for Windows.
- slightly modified the compilation process, extern "ABI" can't be
  customized within Rust so we let Makefile to do the edit.