Fix a regression from "bb496ea8 - server: Always queue mouse messages delivered to another window."

Fix ETHER VAPOR Remaster (214570) launches to black screen when the cursor is in the game window.

The game calls ReleaseCapture() when handling WM_MOUSEMOVE. After bb496ea8, WM_MOUSEMOVE is always
queued because the message window is NULL. So ReleaseCapture() ends up queuing another WM_MOUSEMOVE.
So the game ends up handling infinite WM_MOUSEMOVE messages at startup and is not able to do anything.