GdkWin32GLContext: Check availability of EGL at runtime

Merged Luca Bacci requested to merge lb90/gtk:check-egl-support-at-runtime into gtk-3-24

In addition to the build-time check EGL support in libepoxy, check at runtime whether the system actually provides EGL, otherwise libepoxy will return NULL function pointers and GTK will crash. Notes:

  • WGL may be unavailable on some Windows Embedded environments, or in Windows on ARM64
  • EGL may be unavailable if libangle is not deployed with the application. Additionally, we may want to support native EGL on Windows by means of MESA (though I believe it uses WGL, so not sure if it's worth using that), which is not always available

This needs a review by @fanc999 🙂

Edited by Luca Bacci

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