Add a way to delay snapshots in reftests

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.

testsuite/reftests/Makefile.am

@@ -28,7 +28,8 @@ gtk_reftest_LDADD = \
 gtk_reftest_SOURCES = \
 	reftest-module.c		\
 	reftest-module.h		\
-	gtk-reftest.c
+	gtk-reftest.c			\
+	gtk-reftest.h
 
 clean-local:
 	rm -rf output/ || true