Migrate to NanoGUI / Eigen for GUI and linear algebra.

Also introduce a GUI play/pause button.

vis/include/util.hpp

@@ -1,13 +1,13 @@
 #pragma once
 
+#include <nanogui/opengl.h>
+
 #include <cerrno>
 #include <fstream>
 #include <sstream>
 #include <string>
 #include <vector>
 
-#include <glad/glad.h>
-
 std::string utilInfo() {
     std::stringstream ss;
     ss
@@ -19,6 +19,10 @@ std::string utilInfo() {
     return ss.str();
 }
 
+std::string utilGetString(GLenum name) {
+    return reinterpret_cast<char const*>(glGetString(name));
+}
+
 std::string utilReadFile(const std::string &filename) {
     std::ifstream in(filename, std::ios::in | std::ios::binary);
     if (in) {