Fix missing assignment operators in mtl::vecX

Assignment operators for taking base mtl::vec<> template were missing.
This had the effect that an instance of mtl::vec2 could not be assigned
an instance of mtl::vec<2>, for example.

include/mtl/vec.hpp

@@ -159,6 +159,11 @@ public:
 		x = _x;
 		y = _y;
 	}
+
+	constexpr vec2& operator=(const vec<2>& other) noexcept {
+		vec::operator=(other);
+		return *this;
+	}
 };
 
 class vec3 : public vec<3> {
@@ -174,6 +179,11 @@ public:
 		y = _y;
 		z = _z;
 	}
+
+	constexpr vec3& operator=(const vec<3>& other) noexcept {
+		vec::operator=(other);
+		return *this;
+	}
 };
 
 class vec4 : public vec<4> {
@@ -191,6 +201,11 @@ public:
 		z = _z;
 		w = _w;
 	}
+
+	constexpr vec4& operator=(const vec<4>& other) noexcept {
+		vec::operator=(other);
+		return *this;
+	}
 };
 
 template <typename STREAM_TYPE, size_t N>