Separate position correction for vision estimate out from GPS logic. Add weight parameters. Fixes valid position indication.

I have added three parameters that the vision estimate was sharing with GPS. 
INAV_W_Z_VIS_P
INAV_W_XY_VIS_P
INAV_W_XY_VIS_V
This is a typo. Good catch, thanks. This is a immitating the same logic for gps. I don't use this feature because ar_track_alvar doesn't provide velocity estimate. Should I leave this in (once fixed). I don't know if we will have a use for vision velocity estimate?