adds flags to allow the creation of shared rendertargets

This changes the meaning of allowShared in the creation of RenderTargets which seemed to not be used anywhere in the code from `ResourceOptionFlags.KeyedMutex` to `ResourceOptionFlags.Shared` allowing to share the texture between different contexts. I'm sending another PR to vvvv-dx11 which complements this allowing to create a TempTargetRenderer which uses shared render targets