Revert "Matrix4::getIdentity() implemented by Eigen"

For some reason this broke the rendering of 2D curves.

This reverts commit b8d1012.

libs/math/Matrix4.cpp

@@ -48,6 +48,18 @@ Matrix4::Matrix4(double xx_, double xy_, double xz_, double xw_,
 
 // Named constructors
 
+// Identity matrix
+const Matrix4& Matrix4::getIdentity()
+{
+    static const Matrix4 _identity(
+        1, 0, 0, 0,
+        0, 1, 0, 0,
+        0, 0, 1, 0,
+        0, 0, 0, 1
+    );
+    return _identity;
+}
+
 // Get a rotation from 2 vectors (named constructor)
 Matrix4 Matrix4::getRotation(const Vector3& a, const Vector3& b)
 {

libs/math/Matrix4.h

@@ -72,10 +72,7 @@ class alignas(16) Matrix4
     const Eigen::Projective3d& eigen() const { return _transform; }
 
     /// Obtain the identity matrix.
-    static Matrix4 getIdentity()
-    {
-        return Matrix4(Eigen::Projective3d::Identity());
-    }
+    static const Matrix4& getIdentity();
 
     /// Get a matrix representing the given 3D translation.
     static Matrix4 getTranslation(const Vector3& tr)

libs/transformlib.h

@@ -13,8 +13,7 @@ class IdentityTransform :
 	/// \brief Returns the identity matrix.
 	const Matrix4& localToParent() const
 	{
-		static const Matrix4 ID = Matrix4::getIdentity();
-        return ID;
+		return Matrix4::getIdentity();
 	}
 };