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.
10 jobs for master in 40 minutes and 38 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #186510
build

00:06:04

passed #186515
build_android

00:02:04

passed #186511
build_clang_libcpp

00:06:04

passed #186514
build_clazy

00:04:46

passed #186512
build_gcc_49

00:10:05

passed #186513
build_mingw64_fedora

00:05:48

 
  Document
passed #186517
cpp_docs

00:05:55

passed #186518
glib_docs

00:06:06

passed #186516
qt5_docs

00:05:24

 
  Publish
passed #186519
trigger_pages

00:00:26