Allow other routines to increment/decrement goal_view_offset while keeping it

in the allowable 0-360 range.

src/Main/fg_props.cxx

@@ -180,6 +180,17 @@ getGoalViewOffset ()
 static void
 setGoalViewOffset (double offset)
 {
+    while ( offset < 0 ) {
+	offset += 360.0;
+    }
+    while ( offset > 360.0 ) {
+	offset -= 360.0;
+    }
+    // Snap to center if we are close
+    if ( fabs( offset ) < 1.0 ) {
+	offset = 0.0;
+    }
+
     globals->get_current_view()
 	->set_goal_view_offset(offset * SGD_DEGREES_TO_RADIANS);
 }

src/Main/viewer.hxx

@@ -142,6 +142,12 @@ public:
     inline void set_goal_view_offset( double a) {
 	set_dirty();
 	goal_view_offset = a;
+	while ( goal_view_offset < 0 ) {
+	    goal_view_offset += 360.0;
+	}
+	while ( goal_view_offset > 360.0 ) {
+	    goal_view_offset -= 360.0;
+	}
     }
     inline void set_reverse_view_offset( bool val ) {
 	reverse_view_offset = val;