update parallel rdp

parallel-rdp-standalone/glguts.c

@@ -363,6 +363,8 @@ void screen_init()
     glBufferStorage(GL_PIXEL_UNPACK_BUFFER, buffer_size * TEX_NUM, 0, GL_MAP_WRITE_BIT | GL_MAP_PERSISTENT_BIT | GL_MAP_COHERENT_BIT);
     buffer_data = glMapBufferRange(GL_PIXEL_UNPACK_BUFFER, 0, buffer_size * TEX_NUM, GL_MAP_WRITE_BIT | GL_MAP_PERSISTENT_BIT | GL_MAP_COHERENT_BIT);
 
+    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
+
     // check if there was an error when using any of the commands above
     gl_check_errors();
 }