Fix X11 memoryleak for round corners

When using round corners, the shape_gc in x_win_round_corners never got
freed. This created a memory leak in x11 itself and let the X process
create more load while becoming sluggish in some situations.

From the docs, XFreeGC actually could return an int and could throw a
BadGC error. Neither of them are important to catch in our situation:

- Having a look into the Xlib sourcecode, XFreeGC returns always 1.
- As we don't overwrite the shape_gc or do any other destoying calls,
  the BadGC can't happen in this code.

src/x11/x.c

@@ -141,6 +141,7 @@ static void x_win_round_corners(window_x11 *win, const int rad)
 
         XShapeCombineMask(xctx.dpy, win->xwin, ShapeBounding, 0, 0, mask, ShapeSet);
 
+        XFreeGC(xctx.dpy, shape_gc);
         XFreePixmap(xctx.dpy, mask);
 
         XShapeSelectInput(xctx.dpy,