Add glfwGetEGLConfig native access function

This adds the glfwGetEGLConfig function for querying the EGLConfig of
the EGLSurface of a window.

This is a re-implementation of the PR #2045 by knokko, slightly
redesigned to handle EGLConfig being an opaque type in core EGL.

Closes #2045

CONTRIBUTORS.md

@@ -124,6 +124,7 @@ video tutorials.
  - Josh Kilmer
  - Byunghoon Kim
  - Cameron King
+ - knokko
  - Peter Knut
  - Christoph Kubisch
  - Yuri Kunde Schlesner