The fix in commit 5e7f227d broke shadows while trying to make them
faster.
So use a better way to make them faster.
With the normalized blur radius, we can now conclude that all the values
too far from p.y will cause the gauss() call to return close to 0, so we
can skip any y value that is too far from p.y.
And that allows us to put an upper limit on the loop iterations.
Tests included
Fixes#6888
cc3ed89e34