egl: Move old EGL_MESA_screen_surface spec

It was the lone file in src/egl/docs. Move it to where the other specs
live, in $MESA_TOP/docs/specs.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

diff --git a/src/egl/docs/EGL_MESA_screen_surface b/docs/specs/OLD/EGL_MESA_screen_surface.txt
similarity index 100%
rename from src/egl/docs/EGL_MESA_screen_surface
rename to docs/specs/OLD/EGL_MESA_screen_surface.txt