use int for callbacks; use corect backspace code

Former-commit-id: b86117ea061aec3f88a85f2e8ec76884191b2d38

include/igl/opengl/glfw/Viewer.h

@@ -142,8 +142,8 @@ namespace glfw
     std::function<bool(Viewer& viewer, float delta_y)> callback_mouse_scroll;
     std::function<bool(Viewer& viewer, unsigned int key, int modifiers)> callback_key_pressed;
     // THESE SHOULD BE DEPRECATED:
-    std::function<bool(Viewer& viewer, unsigned char key, int modifiers)> callback_key_down;
-    std::function<bool(Viewer& viewer, unsigned char key, int modifiers)> callback_key_up;
+    std::function<bool(Viewer& viewer, unsigned int key, int modifiers)> callback_key_down;
+    std::function<bool(Viewer& viewer, unsigned int key, int modifiers)> callback_key_up;
     // Pointers to per-callback data
     void* callback_init_data;
     void* callback_pre_draw_data;

tutorial/107_MultipleMeshes/main.cpp

@@ -1,5 +1,6 @@
 #include "tutorial_shared_path.h"
 #include <igl/opengl/glfw/Viewer.h>
+#include <GLFW/glfw3.h>
 #include <string>
 #include <iostream>
 
@@ -26,8 +27,7 @@ int main(int argc, char * argv[])
   viewer.callback_key_down = 
     [&](igl::opengl::glfw::Viewer &, unsigned int key, int mod)
   {
-    // Delete
-    if(key == 3)
+    if(key == GLFW_KEY_BACKSPACE)
     {
       viewer.erase_mesh(viewer.selected_data_index);
       return true;