main: treat elevation offset as offset from horizon

Rather than having somewhat confusing flipped logic when the rotator is
operating in the 180-90 degree elevation regime, this internally
performs sign flipping. This means that a 3 degree elevation offset
means "point an additional 3 degrees up from the horizon" regardless
of whether the rotator is operating in the 0-90 or 180-90 elevation
ranges.

src/main.zig

@@ -182,7 +182,7 @@ pub const RotInt = struct {
         if (self.pollcount == 0) {
             var mangled: AzEl = .{
                 .az = self.requested_posture.az + self.offsets.az,
-                .el = self.requested_posture.el + self.offsets.el,
+                .el = self.requested_posture.el + if (self.requested_posture.el > 90) -self.offsets.el else self.offsets.el,
             };
             mangled.el = if (mangled.el > 90)
                 @min(mangled.el, 180 - self.conf.elevation_mask)