Simplify Vector3 addition operators

Operators are now non-member functions, and no longer have a separate template
type for the second vector (this would prevent adding a BasicVector3<double> to
a BasicVector3<float>, but it does not seem that we have any code which needs
to do this).

libs/math/Vector3.h

@@ -116,26 +116,6 @@ class BasicVector3
         return BasicVector3<T>(-_v[0], -_v[1], -_v[2]);
     }
 
-    /*  Define the addition operators + and += with any other BasicVector3 of type OtherElement
-     *  The vectors are added to each other element-wise
-     */
-    template<typename OtherElement>
-    BasicVector3<T> operator+ (const BasicVector3<OtherElement>& other) const {
-        return BasicVector3<T>(
-            _v[0] + static_cast<T>(other.x()),
-            _v[1] + static_cast<T>(other.y()),
-            _v[2] + static_cast<T>(other.z())
-        );
-    }
-
-    template<typename OtherElement>
-    BasicVector3<T>& operator+= (const BasicVector3<OtherElement>& other) {
-        _v[0] += static_cast<T>(other.x());
-        _v[1] += static_cast<T>(other.y());
-        _v[2] += static_cast<T>(other.z());
-		return *this;
-    }
-
     /*  Define the substraction operators - and -= with any other BasicVector3 of type OtherElement
      *  The vectors are substracted from each other element-wise
      */
@@ -443,6 +423,23 @@ BasicVector3<T>& operator*=(BasicVector3<T>& v, S other)
     return v;
 }
 
+/// Componentwise addition of two vectors
+template <typename T>
+BasicVector3<T> operator+(const BasicVector3<T>& v1, const BasicVector3<T>& v2)
+{
+    return BasicVector3<T>(v1.x() + v2.x(), v1.y() + v2.y(), v1.z() + v2.z());
+}
+
+/// Componentwise vector addition in place
+template <typename T>
+BasicVector3<T>& operator+=(BasicVector3<T>& v1, const BasicVector3<T>& v2)
+{
+    v1.x() += v2.x();
+    v1.y() += v2.y();
+    v1.z() += v2.z();
+    return v1;
+}
+
 /// Stream insertion for BasicVector3
 template<typename T>
 inline std::ostream& operator<<(std::ostream& st, BasicVector3<T> vec)

test/math/Vector.cpp

@@ -88,17 +88,33 @@ TEST(MathTest, NormaliseVector3)
 TEST(MathTest, AddVector3)
 {
     Vector3 v1(2, -5, 17);
-    Vector3 v2(11, 12, -0.5);
+    const Vector3 v1Orig = v1;
+    const Vector3 v2(11, 12, -0.5);
 
-    EXPECT_EQ(v1 + v2, Vector3(13, 7, 16.5));
+    // Add and return
+    const Vector3 sum = v1 + v2;
+    EXPECT_EQ(sum, Vector3(13, 7, 16.5));
+    EXPECT_EQ(v1, v1Orig);
+
+    // Add in place
+    v1 += v2;
+    EXPECT_EQ(v1, sum);
 }
 
 TEST(MathTest, AddVector4)
 {
     Vector4 v1(10, 0.25, -60, 11);
-    Vector4 v2(0, -18, 276, 0.75);
+    const Vector4 v1Orig = v1;
+    const Vector4 v2(0, -18, 276, 0.75);
+
+    // Add and return
+    const Vector4 sum = v1 + v2;
+    EXPECT_EQ(sum, Vector4(10, -17.75, 216, 11.75));
+    EXPECT_EQ(v1, v1Orig);
 
-    EXPECT_EQ(v1 + v2, Vector4(10, -17.75, 216, 11.75));
+    // Add in place
+    v1 += v2;
+    EXPECT_EQ(v1, sum);
 }
 
 TEST(MathTest, SubtractVector3)