Add simdMat4 static constants

- Tests, too

glm/gtx/simd_mat4.hpp

@@ -83,6 +83,11 @@ namespace detail
 			static GLM_RELAXED_CONSTEXPR precision prec = defaultp;
 #		endif//GLM_META_PROG_HELPERS
 
+#		ifdef GLM_STATIC_CONST_MEMBERS
+			static const type ZERO;
+			static const type IDENTITY;
+#		endif
+
 		GLM_FUNC_DECL length_t length() const;
 
 		fvec4SIMD Data[4];

glm/gtx/simd_mat4.inl

@@ -61,6 +61,11 @@ GLM_FUNC_QUALIFIER fvec4SIMD const & fmat4x4SIMD::operator[]
 	return this->Data[i];
 }
 
+#ifdef GLM_STATIC_CONST_MEMBERS
+	const fmat4x4SIMD fmat4x4SIMD::ZERO(0);
+	const fmat4x4SIMD fmat4x4SIMD::IDENTITY(1);
+#endif
+
 //////////////////////////////////////////////////////////////
 // Constructors