Support input gain and output volume controls

[mikeoliphant]

Add control ports for input gain and output volume.

[mikeoliphant]

This is mostly resolved, but since it is using LV2 control ports, it per-sample-buffer and not per-sample accurate. Using LV2 parameters would be better.

[falkTX]

hosts will split the audio cycle into multiple parts as needed, so lv2 params is not always necessary. some parameter smoothing is likely to be enough here, I dont suppose many people will be automating these specific controls