egl: implement EGL_KHR_display_reference on all platforms

https://www.khronos.org/registry/EGL/extensions/KHR/EGL_KHR_display_reference.txt

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@lynxeye I saw on IRC that you were interested in this extension, so I dusted off and rebased this old (!) branch of mine.
Not tested yet, but maybe you'll do that for me 😜
Tested using this new piglit test.