xcbgrab: Use the correct geometry for the region highlight

The feature is implemented using a transparent window and drawing
inside it a rectangle filling the whole window to highlight it.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavdevice/xcbgrab.c

@@ -566,7 +566,7 @@ 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);