eglut: introduce eglut_wsi_interface
The goal is to unify the *_x11 and *_wayland binaries into one, just like how the Vulkan demo does it. Bonus: The OpenGLESv1 demos now run on Wayland.
The last commit fixes a preexisting bug that I discovered when I was playing around with the OpenGLESv1 demos.
Edited by He Haocheng