picom: fix binding root back pixmap in case of depth mismatch

the root back pixmap's depth may mismatch the display's screen depth. in
this case picom will fail to bind it what causes errors, warnings and
weird behavior. instead of using the display's screen depth we will use
the root back pixmap's depth.

src/picom.c

@@ -991,15 +991,31 @@ void root_damaged(session_t *ps) {
 		if (ps->root_image) {
 			ps->backend_data->ops->release_image(ps->backend_data, ps->root_image);
 		}
+
 		auto pixmap = x_get_root_back_pixmap(ps->c, ps->root, ps->atoms);
 		if (pixmap != XCB_NONE) {
+			xcb_generic_error_t *e;
+			auto r = xcb_get_geometry_reply(
+			    ps->c, xcb_get_geometry(ps->c, pixmap), &e);
+			if (!r) {
+				x_print_error(e->full_sequence, e->major_code,
+				              e->minor_code, e->error_code);
+				free(e);
+				goto err;
+			}
+
 			ps->root_image = ps->backend_data->ops->bind_pixmap(
-			    ps->backend_data, pixmap, x_get_visual_info(ps->c, ps->vis), false);
+			    ps->backend_data, pixmap,
+			    x_get_visual_info(ps->c, x_get_visual_for_depth(ps->c, r->depth)),
+			    false);
+			free(r);
+
 			if (ps->root_image) {
 				ps->backend_data->ops->set_image_property(
 				    ps->backend_data, IMAGE_PROPERTY_EFFECTIVE_SIZE,
 				    ps->root_image, (int[]){ps->root_width, ps->root_height});
 			} else {
+			err:
 				log_error("Failed to bind root back pixmap");
 			}
 		}