Examples: GLFW* Make subsequent window creation possible.

examples/opengl2_example/imgui_impl_glfw_gl2.cpp

@@ -233,6 +233,7 @@ static void ImGui_ImplGlfw_InstallCallbacks(GLFWwindow* window)
 bool    ImGui_ImplGlfwGL2_Init(GLFWwindow* window, bool install_callbacks)
 {
     g_Window = window;
+    g_Time = 0;
 
     // Setup back-end capabilities flags
     ImGuiIO& io = ImGui::GetIO();

examples/opengl3_example/imgui_impl_glfw_gl3.cpp

@@ -358,6 +358,7 @@ static void ImGui_ImplGlfw_InstallCallbacks(GLFWwindow* window)
 bool    ImGui_ImplGlfwGL3_Init(GLFWwindow* window, bool install_callbacks, const char* glsl_version)
 {
     g_Window = window;
+    g_Time = 0;
 
     // Store GL version string so we can refer to it later in case we recreate shaders.
     if (glsl_version == NULL)