Rename weston_compositor EGLDisplay member to egl_display

Rename weston_compositor EGLDisplay member to egl_display

EGLDisplay is helpfully typedeffed as void *, which means that you won't
get conflicting-pointer-type warnings if you accidentally confuse it
with weston_compositor::wl_display.  Rename it to make it more clear
which display you're dealing with, and also rename compositor-wayland's
parent.display member to parent.wl_display.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>