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.
6 jobs for master in 58 minutes and 59 seconds (queued for 56 seconds)
Status Job ID Name Coverage
  Build
passed #72033
build

00:04:50

passed #72038
build_android

00:01:58

passed #72034
build_clang_libcpp

00:04:28

passed #72037
build_clazy

00:04:17

passed #72035
build_gcc_49

00:05:03

passed #72036
build_mingw64_fedora

00:38:20