don't wait for windows to be mapped to send their contents

[totaam]

At the moment, for non override-redirect windows, when a new window is created we have to wait for the client to also create the remote window, it then sends the geometry used back to the server which can then map the window and sends the contents.
Over a slow connection, this causes unnecessary delays during which the window contents are blank.

Instead, we can pre-map the window where we think it is going to land on screen (where the application requested it) and send the contents immediately - if the client maps it in a different location, the map-window packet and configure-window will adjust the window location later.
And if the window is a different size, we will re-send the correct contents - at least there is something to show before that happens.

[totaam]

Seems to work OK!

[totaam]

As of dc13263, to make it easier to trigger the race condition client side, set XPRA_SHOW_DELAY=1000 to delay showing the window by 1000ms. This ensures that draw packets are received before the window is mapped.