reftest: Clean up reftest_inhibit_snapshot()

reftest: Clean up reftest_inhibit_snapshot()

This was done in a weird way where we always call reftest_uninhibit_snapshot()
on paint, and then re-inhibited it if it wasn't inhibited. To make this
work it also started with an extra inhibit.

This is very contorted and based on how this historically worked. This
changes it to just do:

  if (inhibit_count > 0)
    return;

And keep inhibit_count at its initial zero value unless it is actually
inhibited.