Fix crash in Echo effect when changing samplerate

Re-applied c6051e76b46de1485b70d71d0f6ab2c96aab61ce:

As ZynAddSubFX still uses the horrible approach of storing global
settings (such as the currently used samplerate) in global static
variables the Echo effect will crash when the samplerate is changed
for a previously loaded ZynAddSubFX instance.

As a quick fix the original samplerate used for instantiating and
accessing internal arrays (whose size depends on global samplerate
variable) is saved for each instance and used everywhere instead of
synth->samplerate.

Fixes crash when exporting e.g. Saber-FinalStep.mmpz at a higher
samplerate than LMMS normally runs with.