add combineBlend option to HDRStage

If enabled (default: false) the stage will enable alpha blending to
write to the destination frame buffer - allows drawing a scene with HDR
stage on top of something else.
fixed: compile/install issues
fixed: opt build (propagate params)
changed: some cmake clean up
fixed: adjust to vars used by configure packages
changed: cmake cleanup
added  : package awareness (fcd, config.h configured.h)