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>

meson.build

@@ -349,6 +349,8 @@ libi3srcs = [
   'libi3/string.c',
   'libi3/ucs2_conversion.c',
   'libi3/nonblock.c',
+  'libi3/screenshot_wallpaper.c',
+  'libi3/is_background_set.c',
 ]
 
 if not cdata.get('HAVE_STRNDUP')