x11: switch back to StaticGravity

This was changed 6 years ago (444e583b6) and seemed to work fine. But it
does seem to cause issues with IceWM sometimes, while with StaticGravity
the problem is gone. Comparing both gravity values, reading the confused
source code comment, and reading the referenced commit message, I can't
determine what it even does, I just remove it.

Reproduction:
- start mpv in windowed mode, with 2 videos of different size
- switch to second video
- switch window with alt+tab
- switch back to mpv with alt+tab
- window moves to X=0

There's probably a better way to fix this. Please send a patch.

video/out/x11_common.c

@@ -1308,11 +1308,8 @@ static void vo_x11_sizehint(struct vo *vo, struct mp_rect rc, bool override_pos)
     hint->flags |= PMinSize;
     hint->min_width = hint->min_height = 4;
 
-    // This will use the top/left corner of the window for positioning, instead
-    // of the top/left corner of the client. _NET_MOVERESIZE_WINDOW could be
-    // used to get a different reference point, while keeping gravity.
     hint->flags |= PWinGravity;
-    hint->win_gravity = CenterGravity;
+    hint->win_gravity = StaticGravity;
 
     XSetWMNormalHints(x11->display, x11->window, hint);
     XFree(hint);