COFC: fix cam jumping when zooming off-map (when using ZoomTilt, but not-Freemode) by removing extra unnecessary off-map boundary check

by using ExtendedGroundHeight() function:
-a bit improve off-map zoom precision (when using Freemode, but not-ZoomTilt) 
-reduce sliding when zooming off-map/map corner (when using ZoomTilt and Freemode) (but still can't pin-point cause)
tweaks:
-a bit increase zoom precision when using ZoomTilt (increase some looping to find ground height, only for ZoomTilt)
Merge pull request #257 from xponen/patch-8

COFC: fix cam jumping when zooming off-map (when using ZoomTilt, but not -Freemode) by removing extra unnecessary off-map boundary check