prevent flickering when changing slides

When the window background is left at WhitePixel (from initialization) and the
user has set a dark background color in config.h, the window shortly displays
the white default background when clearing after advancing to another slide.
This was very disturbing and is fixed now by setting the default window
background color to the bgcol chosen in config.h.

sent.c

@@ -572,14 +572,13 @@ void xinit()
 	xw.vis = XDefaultVisual(xw.dpy, xw.scr);
 	resize(DisplayWidth(xw.dpy, xw.scr), DisplayHeight(xw.dpy, xw.scr));
 
-	xw.attrs.background_pixel = WhitePixel(xw.dpy, xw.scr);
 	xw.attrs.bit_gravity = CenterGravity;
 	xw.attrs.event_mask = KeyPressMask | ExposureMask | StructureNotifyMask
 		| ButtonMotionMask | ButtonPressMask;
 
 	xw.win = XCreateWindow(xw.dpy, XRootWindow(xw.dpy, xw.scr), 0, 0,
 			xw.w, xw.h, 0, XDefaultDepth(xw.dpy, xw.scr), InputOutput, xw.vis,
-			CWBackPixel | CWBitGravity | CWEventMask, &xw.attrs);
+			CWBitGravity | CWEventMask, &xw.attrs);
 
 	xw.wmdeletewin = XInternAtom(xw.dpy, "WM_DELETE_WINDOW", False);
 	xw.netwmname = XInternAtom(xw.dpy, "_NET_WM_NAME", False);
@@ -589,6 +588,7 @@ void xinit()
 		die("Can't create drawing context.");
 	sc = drw_scm_create(d, fgcol, bgcol);
 	drw_setscheme(d, sc);
+	XSetWindowBackground(xw.dpy, xw.win, sc->bg.pix);
 
 	xloadfonts();