Refactor simulate to split UI from application

Fix name of glew library

Move simulate to new directory and refactor to use C++ conventions

Wrap simulate in mujoco namespace

fix namespaces, convert struct to class, use std::lock_guard

use GetMutex to better handle destruction ordering

rename mjSimulate to Simulate

nit: space before public and typo

move startup print

use nullptr instead of NULL

use GetInstance() to get simulate object

convert init to default constructor

simulate/uitools.h

@@ -0,0 +1,65 @@
+// Copyright 2021 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_UITOOLS_H_
+#define MUJOCO_UITOOLS_H_
+
+
+#include <GLFW/glfw3.h>
+#include <mujoco/mujoco.h>
+
+
+// this is a C-API
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+
+// User-supplied callback function types.
+typedef void (*uiEventFn)(mjuiState* state);
+typedef void (*uiLayoutFn)(mjuiState* state);
+typedef void (*uiRenderFn)(mjuiState* state);
+typedef void (*uiDropFn)  (mjuiState* state, int count, const char** paths);
+
+// Container for GLFW window pointer.
+struct _uiUserPointer {
+  mjuiState* state;
+  uiEventFn uiEvent;
+  uiLayoutFn uiLayout;
+  uiRenderFn uiRender;
+  uiDropFn   uiDrop;
+  double buffer2window;
+};
+typedef struct _uiUserPointer uiUserPointer;
+
+// Set internal and user-supplied UI callbacks in GLFW window.
+void uiSetCallback(GLFWwindow* wnd, mjuiState* state,
+                   uiEventFn uiEvent, uiLayoutFn uiLayout,
+                   uiRenderFn uiUserRender, uiDropFn uiUserDrop);
+
+// Clear UI callbacks in GLFW window.
+void uiClearCallback(GLFWwindow* wnd);
+
+// Compute suitable font scale.
+int uiFontScale(GLFWwindow* wnd);
+
+// Modify UI structure.
+void uiModify(GLFWwindow* wnd, mjUI* ui, mjuiState* state, mjrContext* con);
+
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif  // MUJOCO_UITOOLS_H_