common: fix bezAngleAt function

In the function calculating the tangent to a cubic bezier
at a given t, the atan was calculated using dx over dy,
instead of dy over dx. The error was visible during
the animation using auto-orient = true.

src/common/tvgLines.cpp

@@ -238,7 +238,7 @@ float bezAngleAt(const Bezier& bz, float t)
     pt.x *= 3;
     pt.y *= 3;
 
-    return mathRad2Deg(atan2(pt.x, pt.y));
+    return mathRad2Deg(atan2(pt.y, pt.x));
 }