#44 parallel plugin output

[Craig Stuart Sapp]

Vamp plugins calculate all outputs in parallel.
For example if a plugin has two output channels,
both are calculated whenever the plugin is called.

In Sonic Visualiser, only one output is selectable
at a time. For example, if I want to display the
two outputs from a plugin, I would call the plugin
from SV once to get the first output, and the
second output goes off to /dev/null.
Then I call the plugin again to get the second output,
and this time the first output from the plugin
goes to /dev/null.

However, it would be useful to just need to run
the plugin once and not recalculate the data for the
second output by calling the plugin again.