Added Vector::lengthSquared() function.

The name is a little bit misleading, but it is for more efficient vector
length comparisons.

src/Math/Test/VectorTest.cpp

@@ -93,6 +93,10 @@ void VectorTest::length() {
     QCOMPARE(Vector4(1.0f, 2.0f, 3.0f, 4.0f).length(), 5.4772256f);
 }
 
+void VectorTest::lengthSquared() {
+    QCOMPARE(Vector4(1.0f, 2.0f, 3.0f, 4.0f).lengthSquared(), 30.0f);
+}
+
 void VectorTest::normalized() {
     QVERIFY(Vector4(1.0f, 1.0f, 1.0f, 1.0f).normalized() == Vector4(0.5f, 0.5f, 0.5f, 0.5f));
 }

src/Math/Test/VectorTest.h

@@ -30,6 +30,7 @@ class VectorTest: public QObject {
         void multiplyDivide();
         void addSubstract();
         void length();
+        void lengthSquared();
         void normalized();
         void product();
         void angle();

src/Math/Vector.h

@@ -218,11 +218,27 @@ template<class T, size_t size> class Vector {
             return out;
         }
 
-        /** @brief %Vector length */
+        /**
+         * @brief %Vector length
+         *
+         * @see lengthSquared()
+         */
         inline T length() const {
             return sqrt(dot(*this, *this));
         }
 
+        /**
+         * @brief %Vector length squared
+         *
+         * More efficient than length() for comparing vector length with
+         * other values, because it doesn't compute the square root, just the
+         * dot product: @f$ a \cdot a < length \cdot length @f$ is faster
+         * than @f$ \sqrt{a \cdot a} < length @f$.
+         */
+        inline T lengthSquared() const {
+            return dot(*this, *this);
+        }
+
         /** @brief Normalized vector (of length 1) */
         inline Vector<T, size> normalized() const {
             return *this/length();