[filter_ccs]: check variance on eroded image

Often, we see text that also has stray lines connected to it in its
component. By eroding the component before checking variance, we only
look at the variance of the core of the object.

This may give more false positives theoretically, but works well
practically.
[swt]: use Sobel, smaller blur

I can't come up with an explanation for why Sobel works better; it seems
Canny's "continuous line" hysteresis is screwing us over. Either way,
Sobel works, so I'll keep it.
[filter_ccs]: tune thresholds