v3d: Only add output slot tracking for the current varying slot.

v3d: Only add output slot tracking for the current varying slot.

We always emit 4 slots per slot because things like color output and
position processing in the epilogue will potentially look up more values
than the variable declaration had.  However, when we get a .location_frac
!= 0, we don't want to overwrite components of the following
.driver_location.