This patch uses the BKE implicit conversion rules to implement implicit
conversion in multi-function procedure operations in the compositor.
Since conversions use ColorSceneLinear4f to represent colors instead of
float4, we need to insert extra implicit conversions around color
sockets.
Special attention was given to variable destruction in the
implementation because it is now possible for implicit variables to be
outputs, so we need to make sure they are not destructed.
27bf89e933