Do not "set" the wallpaper during startup (#4373)

"Set" the wallpaper during startup only sometimes

Since commit 4f5e0e7, i3 would take a screenshot and set that as the
background pixmap of the root window during startup. This is the easy
part of setting a proper X11 wallpaper.

The code in question was added because something either set the
background pixmap of the root window to NONE or the X11 server was
started with "-background none". This is apparently done by default by
e.g. gdm to avoid some flickering while the X11 server starts up.

This commit makes this code conditional: Only when no wallpaper is
detected is a screenshot taken.

Since I could not find any way to query the background of a window, a
more direct approach is taken to detect this situation: First, we find
some part of the root window that is not currently covered. Then we open
a white window there, close it again and grab a screenshot. If a
wallpaper is set, the X11 server will draw this wallpaper after the
window is closed and something else will be visible in the screenshot.

However, the wallpaper could have a white pixel at the tested position.
Thus, this procedure is repeated with a black window.

Only when this procedure produces two different pixel values is a
screenshot taken and set as the wallpaper.

Fixes: https://github.com/i3/i3/issues/4371
Fixes: https://github.com/i3/i3/issues/2869
Signed-off-by: Uli Schlachter <psychon@znc.in>

include/libi3.h

@@ -655,3 +655,16 @@ int create_socket(const char *filename, char **out_socketpath);
  *
  */
 bool path_exists(const char *path);
+
+/**
+ * Grab a screenshot of the screen's root window and set it as the wallpaper.
+ */
+void set_screenshot_as_wallpaper(xcb_connection_t *conn, xcb_screen_t *screen);
+
+/**
+ * Test whether the screen's root window has a background set.
+ *
+ * This opens & closes a window and test whether the root window still shows the
+ * content of the window.
+ */
+bool is_background_set(xcb_connection_t *conn, xcb_screen_t *screen);