Fix setting antialiasing by editing Cairo context.

Commit 853e9499 added setAntialias() to CairoOutputDev, in particular
to allow setting up antialiasing on the pdftocairo utils through a CLI
option (-antialias).
Unfortunately this broke applications which used the glib API and used
to simply set the antialiasing through the Cairo context, and now always
get the default antialiasing (even if we added a new API, it would break
API behavior and simplicity).

Instead only use the `antialias` member if it is set to non-default,
which means it has been changed explicitly by some means. Keep the cairo
context as-is otherwise.
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