snapshot: Fix assert in inhibition

snapshot: Fix assert in inhibition

In https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/2027 i was getting

Bail out! ERROR:../testsuite/reftests/reftest-snapshot.c:212:reftest_uninhibit_snapshot: assertion failed: (inhibit_count > 0)

In (for example the box-shadow-changes-modify-clip reftest. I can reproduce this (on master) with:

```
$ xvfb-run -a -s "-screen 0 1024x768x24" meson test --suite gtk:reftest "reftest box-shadow-changes-modify-clip.ui"
...
1/1 gtk:reftest / reftest box-shadow-changes-modify-clip.ui ERROR          0.77s
``

Fix this by re-inhibiting if we didn't draw anything, or we will get an assert the next paint.