avdevice/xcbgrab: Fix show_region rectangle

Fixes trac ticket #4164

This is to address an error when using show_region, which would cause part of the captured area to become static.
It looks like the rectangle specifying the capture area was relative to the capture window.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/xcbgrab.c

@@ -564,7 +564,7 @@ static void setup_window(AVFormatContext *s)
     uint32_t values[] = { 1,
                           XCB_EVENT_MASK_EXPOSURE |
                           XCB_EVENT_MASK_STRUCTURE_NOTIFY };
-    xcb_rectangle_t rect = { c->x, c->y, c->width, c->height };
+    xcb_rectangle_t rect = { 0, 0, c->width, c->height };
 
     c->window = xcb_generate_id(c->conn);