This also switches the rendering code from using gsk_render_node_draw()
to gsk_renderer_render_texture().
Some tests are broken with the GL renderer, so this patch forces the
Cairo renderer until they get fixed.
As we are building the gtkreftestprivate and reftest test libraries as
DLLs, we need to export the symbols in there so that things will link.
Decorate the symbols with G_MODULE_EXPORT for this purpose.
