fixed invisible envelope request if request background is masked

When using "global.use_envelope_request: true" (to use envelope style
request dialogs instead of door style request dialogs) together with
"background.request.draw_masked: true" (to use a partially transparent
background image for the request dialog), no request dialog is drawn
at all (so the request dialog has to be answered blindly). This change
fixes this problem.

There are a few graphical bugs left (if the request dialog opens after
a game was lost, asking to play again) which still have to be fixed.