meta/pkg-push: note on cleaning out old package builds

I didn't implement an automatic clean-up, since this service is
going away soon, hopefully.
make installer: prevent pollution from local to catalog-based build
racket/gui win32: enable clipping-problem workaround for 32-bit mode

The specialization to Win64 to work around a Cairo clipping
problem probably had more to do with the version of Cairo,
instead of the variant of Windows, since both variants need the
workarounds after a Cairo upgrade.
racket/gui: use cairo_paint() to copy buffer to screen

Following 54e42664df, use cairo_paint() instead of cairo_fill()
to copy offscreen content onscreen, just in case it's faster.