Add relative mode to EFX - the movement is relative to head's current position

I have made this pre GM - I decided that GM should affect also relative EFX if needed/requested.
It also simplified the code a bit (compared to previous version)

This change allows few things:

- run more effects in parallel on the same movers (e.g. pan saw and tilt saw)

- prepare the efx's in advance without regard of the actual movers' position; then adjust
the position using scene made at the venue (make some efx at home - with relative flag;
at the venue, create scenes where all your head point down/up/left/right/center; now start
e.g. center scene and an efx - the efx will point at the centre; start right scene -
the efx will move to the right, etc.)

- control efx center in runtime by assigning xyarea/sliders to pan/tilt channels and adding relative efx

NOTE: prepare efx with center at 127;127 to be neutral
Small optimisation - do not compute relative values if they were not used