wayland-egl: import libwayland-egl.so frontend library from Mesa

Currently the client-facing libwayland-egl API is defined by a header
file shipped by Wayland, but the implementation is left to each vendor.

This can cause collisions when multiple implementations are installed on
the same system. Importing the implementation into Wayland with a stable
and versioned driver-facing ABI allows multiple drivers to coexist on
the same system.

Pull the sample implementation from Mesa commit 677edff5cfd
("wayland-egl: rework and simplify wl_egl_window initialization")
It has been used by the Mesa open source drivers, NVIDIA and others[1].

v2: Reword commit message, rebase on top of newer Mesa.

[1] https://github.com/thayama/wayland-egl



Cc: Miguel A. Vico <mvicomoya@nvidia.com>
Cc: James Jones <jajones@nvidia.com>
Cc: Daniel Stone <daniels@collabora.com>
Cc: duncan-roe <duncan_roe@optusnet.com.au>
Cc: Takanari Hayama <taki@igel.co.jp>
Suggested-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Miguel A Vico Moya <mvicomoya@nvidia.com>
Reviewed-by: Arnaud Vrac <avrac@freebox.fr>