Implement X11 clipboard using GLFW functions.

src/platform_x11/Clipboard.cpp

-#include <iostream>
+#define GLFW_INCLUDE_NONE
+#include "GLFW/glfw3.h"
+
+#include <string>
+#include <string.h>
 
 namespace Clipboard
 {
   void Copy( const char * data, int len )
   {
-    std::cerr << __FUNCTION__ << " NOT IMPLEMENTED" << std::endl;
+    const std::string contents(data, len);
+    GLFWwindow* window = glfwGetCurrentContext();
+    glfwSetClipboardString(window, contents.c_str());
   }
 
   int GetContentsLength()
   {
-    std::cerr << __FUNCTION__ << " NOT IMPLEMENTED" << std::endl;
-    return 0;
+    GLFWwindow* window = glfwGetCurrentContext();
+    const char* contents = glfwGetClipboardString(window);
+    return strlen(contents);
   }
 
   void GetContents( char * data, int len )
   {
-    std::cerr << __FUNCTION__ << " NOT IMPLEMENTED" << std::endl;
-    data[0] = 0;
+    GLFWwindow* window = glfwGetCurrentContext();
+    const char* contents = glfwGetClipboardString(window);
+    strncpy(data, contents, len);
   }
-
 }