glib: Add poppler_page_set_render_antialias(PopplerPage*, cairo_antialias_t)

The `pdftocairo` command-line tool has an `-antialias` option
that configures the anti-aliasing mode used when rasterizing text and shapes
(for example when the output is PNG). This is based on calling the internal
`CairoOutputDev::setAntialias` method.

In the glib API, `poppler_page_render` is also based on `CairoOutputDev`
but there is no call to `setAntialias` so `CAIRO_ANTIALIAS_DEFAULT`
(set in `CairoOutputDev`’s constructor) is always used regardless
of how the cairo context was configured before being passed to
`poppler_page_render`.

This fills the gap by keeping an antialiasing mode in each `PopplerPage`
object, adding a setter for it, and calling `CairoOutputDev::setAntialias`
in each rendering function.
6 jobs for poppler_page_set_render_antialias in 4 minutes and 21 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #207257
build

00:03:02

passed #207262
build_android

00:01:21

passed #207258
build_clang_libcpp

00:04:21

passed #207261
build_clazy

00:04:12

passed #207260
build_mingw64_fedora

00:02:54

passed #207259
build_ubuntu_16_04

00:02:44