Add static constants for vec2

- Tests, too

glm/detail/type_vec2.hpp

@@ -58,6 +58,10 @@ namespace glm
 			static GLM_RELAXED_CONSTEXPR precision prec = P;
 #		endif//GLM_META_PROG_HELPERS
 
+		static const type ZERO;
+		static const type X;
+		static const type Y;
+		static const type XY;
 		// -- Data --
 
 #		if GLM_HAS_ANONYMOUS_UNION

glm/detail/type_vec2.inl

@@ -28,6 +28,17 @@
 
 namespace glm
 {
+	template <typename T, precision P>
+	const tvec2<T, P> tvec2<T, P>::ZERO = tvec2<T, P>(static_cast<T>(0), static_cast<T>(0));
+
+	template <typename T, precision P>
+	const tvec2<T, P> tvec2<T, P>::X = tvec2<T, P>(static_cast<T>(1), static_cast<T>(0));
+
+	template <typename T, precision P>
+	const tvec2<T, P> tvec2<T, P>::Y = tvec2<T, P>(static_cast<T>(0), static_cast<T>(1));
+
+	template <typename T, precision P>
+	const tvec2<T, P> tvec2<T, P>::XY = tvec2<T, P>(static_cast<T>(1), static_cast<T>(1));
 	// -- Implicit basic constructors --
 
 #	if !GLM_HAS_DEFAULTED_FUNCTIONS || !defined(GLM_FORCE_NO_CTOR_INIT)

test/core/core_type_vec2.cpp

@@ -331,6 +331,17 @@ int test_operator_increment()
 	return Error;
 }
 
+int test_vec2_static_const() {
+	int Error(0);
+
+	Error += (glm::ivec2(0, 0) == glm::ivec2::ZERO) ? 0 : 1;
+	Error += (glm::vec2(1, 0) == glm::vec2::X) ? 0 : 1;
+	Error += (glm::bvec2(false, true) == glm::bvec2::Y) ? 0 : 1;
+	Error += (glm::dvec2(1, 1) == glm::dvec2::XY) ? 0 : 1;
+
+	return Error;
+}
+
 int main()
 {
 	int Error = 0;
@@ -343,6 +354,7 @@ int main()
 		assert(glm::vec2::components == 2);
 #	endif
 
+	Error += test_vec2_static_const();
 	Error += test_vec2_size();
 	Error += test_vec2_ctor();
 	Error += test_vec2_operators();