Following recent Windows versions.

They are not used, and can interfere with other libraries that link
directly to win32u.so.

retina_on was being set (indirectly) by check_retina_status(), but it
relies on retina_on itself.
This resulted in incorrect (non-retina) monitor and work RECTs being
cached, causing odd application bugs like a window that won't resize
larger than 1/4 of the screen.

Initialize retina_on to the value of retina_enabled, so later calls
to check_retina_status() don't result in incorrect data being cached.