Neither Flash or QC can take an image as input in Resolume, so out of those three, FFGL is what you need to look into.
FFGL is a framework that lets you create plugins for a FreeframeGL host. It gives you a set of functions that let you exchange parameter and texture data between your plugin and the host. The framework is based on c++ and OpenGL. FFGL hosts that I currently know of are Resolume, Coge, VDMX and Isadora.
openFrameworks is a framework that lets you create standalone visual applications, based on c++ and OpenGL. It has a huge set of handy functions that take some of the nasty work out of OpenGL programming, but it does not natively run in Resolume.
An FFGL plugin in Resolume can be applied on the clip, layer or composition level. You will receive the output of that comp/layer/clip as a texture and can then further process it. At the end, whatever you draw in your OpenGL context will replace the output of the comp/layer/clip you apply the effect on. Think of it as visual sidechaining.
As long as you want to stay inside of Resolume, FFGL is what you want use. This is what FFGL is designed for.
If your primary interest is getting the output out of Resolume someplace else, you might be better of creating your own app, for instance in openFrameworks. This way you don't have to worry about being a plugin in a host, which may make it difficult to output to external devices. You can then use Spout/Syphon to receive textures from Resolume. Spout (PC)
and Syphon (Mac)
are a way of sharing textures between applications.