WindowServer: Give windows a "background color" to use for missing parts.

When resizing a window, we often end up having to paint some part of it
without coverage in the current backing store. This patch makes those cases
look nicer by having a fallback background color for each window, passed
along with the CreateWindow client message.

Servers/WindowServer/WSAPITypes.h

@@ -221,6 +221,7 @@ struct WSAPI_ClientMessage {
             float opacity;
             WSAPI_Size base_size;
             WSAPI_Size size_increment;
+            WSAPI_Color background_color;
         } window;
         struct {
             WSAPI_Size size;