Implement callback for typed text

Callback is registered to handle characters that user types on
keyboard. For example if shift-2 is pressed on an American
keyboard layout, the text "@" will be generated. Callback is
mapped to 'textinput' event.

lovr-keyboard.lua

@@ -4,10 +4,12 @@ local C = ffi.os == 'Windows' and ffi.load('glfw3') or ffi.C
 ffi.cdef [[
   typedef struct GLFWwindow GLFWwindow;
   typedef void(*GLFWkeyfun)(GLFWwindow*, int, int, int, int);
+  typedef void (*GLFWcharfun)(GLFWwindow*, unsigned int);
 
   GLFWwindow* glfwGetCurrentContext(void);
   int glfwGetKey(GLFWwindow* window, int key);
   GLFWkeyfun glfwSetKeyCallback(GLFWwindow* window, GLFWkeyfun callback);
+  GLFWcharfun glfwSetCharCallback(GLFWwindow* window, GLFWcharfun callback);
 ]]
 
 local window = C.glfwGetCurrentContext()
@@ -148,4 +150,8 @@ C.glfwSetKeyCallback(window, function(window, key, scancode, action, mods)
   end
 end)
 
+C.glfwSetCharCallback(window, function(window, char)
+    lovr.event.push('textinput', string.char(char))
+  end)
+
 return keyboard