Lv2 plugins with no outputs should be loadable

Enhancement Summary

Currently, a few of plugins will not load in LMMS, saying - no audio output channel (if you have LMMS_LV2_DEBUG set).

This is a missed opportunity, because those plugins could be used well.

Implementation Details / Mockup

As favored in discord, since LMMS requires to fill output buffers for all plugins, the solution is to let LMMS fill the output buffer with something sane in Lv2Proc::copyBuffersToCore, even if the plugin does not provide output via an output port. It has been agreed that in that case, the output buffer just shall mirror the values of the input buffer ("pass through").

An implementation must be careful so that for these plugins, the Lv2Proc::outPorts() function will never be called.

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LMMS / lmms