Stop the FFTPassEffect Repeat test after FFT size 128.

The reason is that the 256 test uses texture sizes of 256*31=7936,
and above ~3900, some cards (at least both my Intel and NVidia card)
start having accuracy issues on some sizes. The test happens not to
die on this for semi-obscure reasons, but that's mostly by accident,
and in any case, requiring 8k textures for a unit test might be
a bit on the upper side.