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.
This adds an inhibit api that code from the reftest module
can use to delay the taking of the snapshot. Also refactor
the code in gtk-reftest to use the inhibit mechanism for
its own delaying of the snapshot until after the first
expose.
