This is a massive improvement to stability and safety when using the plugin, as all exceptions should now no longer be leaked into C controlled code, which can't actually handle exceptions at all.
Warnings fixed:
* Potentially throwing exception during library load.
* Possibly throwing function passed to C.
* Statement does nothing.
* Variable is initialized but not referenced.
* Variable overloads variable in parent scope.
This filter allows the use of another source as a mask, allowing complex filter graphs and trippy effects, such as creating a text source with three animated videos, each using a different color channel as the mask.
Fixes#18.
