EGL: Implement EGL_KHR_surfaceless_context Add extension string to registry Add extension flag to EGL and Vulkan Displays Bug: angleproject:3960 Test: dEQP-EGL.functional.surfaceless_context.* angle_end2end_tests --gtest_filter=EGLSurfacelessContextTest* Change-Id: Id17916f1f636454af395b7bc92892dcf8fc93e54 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1885075 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>