GdkWin32GLContext: Check availability of EGL at runtime
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