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
No this is intentional. This logic is redundant in this situation as eph/epv (for vision) is a constant so this logic always comes out to w_gps_xy = 1.  
I left this commented out because this is where we can add varying weight values depending on vision accuracy later on if we decide to do that.