This fixes the case where the effect vanishes, but we still hold an invalid pointer to a parameter of that effect. With the new code, the effect will not vanish as long as an effect pointer exists.
However, all effects must be created either through std::shared_ptr or std::make_shared. If they were not made through one of those means, the code will crash on the ->shared_from_this() call.
When copying or re-ordering filters, an occasional crash can be observed due to the rendering not being possible yet. With this, it will simply revert back to the original source instead of crashing.
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.
