Fix #25: vector:angleTo() returns wrong angle.

vector-light.lua

@@ -121,7 +121,10 @@ local function trim(maxLen, x, y)
 end
 
 local function angleTo(x,y, u,v)
-	return atan2(y - (v or 0), x - (u or 0))
+	if u and v then
+		return atan2(y, x) - atan2(v, u)
+	end
+	return atan2(y, x)
 end
 
 -- the module

vector.lua

@@ -175,8 +175,10 @@ function vector:trim_inplace(maxLen)
 end
 
 function vector:angleTo(other)
-	other = other or zero
-	return atan2(self.y - other.y, self.x - other.x)
+	if other then
+		return atan2(self.y, self.y) - atan2(other.y, other.x)
+	end
+	return atan2(self.y, self.y)
 end
 
 function vector:trimmed(maxLen)