#108 Fix multi-channel LADSPA effects + draining for all LADSPA effects

[spider-mario]

This fixes bug #262 that I submitted (https://sourceforge.net/p/sox/bugs/262/ , LADSPA plugins with more output channels than input channels cause a crash) as well as draining for all LADSPA plugins that have a latency higher than either the buffer size or the input track, in which case it was not completely drained (and the output was thus shorter than the input).

(I discovered the latter issue while trying to fix the former.)

[spider-mario]

Hi again,

I know that LADSPA plugins are not so trendy these days, but now that development appears to have picked up again (thanks!), could we consider merging this? The patch seems to still apply cleanly.