[graph/win32] Implement smooth resizing.
The window creation and the message loop has to be put in a separate thread in order to achieve smooth resizing. Othrewise, the modal loop cannot be interrupted to redraw the canvas. The main thread also has a loop to receive the reposted messages, to update the canvas and send WM_PAINT back via UpdateWindow. After this update the win32 resizing feels similar to the X11 resizing. * graph/win32/grwin32.c (grWin32Surface): Store the host thread id. (gr_win32_surface_init): Spin off `CreateWindow` from here... (Window_ThreadProc): ... to here and run the window message loop. (Message_Process): Updated to repost messages to the host thread. (gr_win32_surface_listen_event): Run the host thread message loop.