Add check to flip quaternion (#1624)

src/raymath.h

@@ -1271,6 +1271,11 @@ RMDEF Quaternion QuaternionSlerp(Quaternion q1, Quaternion q2, float amount)
 
     float cosHalfTheta =  q1.x*q2.x + q1.y*q2.y + q1.z*q2.z + q1.w*q2.w;
 
+    if (cosHalfTheta < 0) {
+        q2.x = -q2.x; q2.y = -q2.y; q2.z = -q2.z; q2.w = -q2.w;
+        cosHalfTheta = -cosHalfTheta;
+    }    
+
     if (fabs(cosHalfTheta) >= 1.0f) result = q1;
     else if (cosHalfTheta > 0.95f) result = QuaternionNlerp(q1, q2, amount);
     else