WindowServer: Add IPC endpoint to get the color under cursor

This allows any client to ask the WindowServer to give it the color
of the screen bitmap under the cursor.

There's currently no way to get the screen bitmap *without* the
cursor already drawn on it, so for now we just take a pixel
beside the actual cursor position to avoid just getting the cursors
color.

Userland/Services/WindowServer/WindowServer.ipc

@@ -136,6 +136,7 @@ endpoint WindowServer
 
     get_screen_bitmap(Optional<Gfx::IntRect> rect, Optional<u32> screen_index) => (Gfx::ShareableBitmap bitmap)
     get_screen_bitmap_around_cursor(Gfx::IntSize size) => (Gfx::ShareableBitmap bitmap)
+    get_color_under_cursor() => (Gfx::Color color)
 
     pong() =|