meson: allow egl_native_platform to be specified

After commit f8dc22bf61, it was no longer possible to have explicitly
enabled platforms with surfaceless being the EGL native platform.  This
fixes that by adding -Degl-native-platform.

Fixes: f8dc22bf61 ("meson: drop deprecated EGL platform build options")
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11992>

meson_options.txt

@@ -27,6 +27,16 @@ option(
   ],
   description : 'window systems to support. If this is set to `auto`, all platforms applicable will be enabled.'
 )
+option(
+  'egl-native-platform',
+  type : 'combo',
+  value : 'auto',
+  choices : [
+    'auto', 'x11', 'wayland', 'haiku', 'android', 'windows',
+    'surfaceless', 'drm',
+  ],
+  description : 'the window system EGL assumes for EGL_DEFAULT_DISPLAY',
+)
 option(
   'android-stub',
   type : 'boolean',