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.