Allow preloading multiple background images to GPU for fast switching

Fixes #9836

tools/cmd/at/set_window_logo.go

@@ -76,3 +76,33 @@ func read_window_logo(io_data *rc_io_data, path string) (func(io_data *rc_io_dat
 		return false, nil
 	}, nil
 }
+
+func is_index_arg(s string) bool {
+	if len(s) == 0 {
+		return false
+	}
+	start := 0
+	if s[0] == '+' || s[0] == '-' {
+		start = 1
+	}
+	if start >= len(s) {
+		return false
+	}
+	for _, c := range s[start:] {
+		if c < '0' || c > '9' {
+			return false
+		}
+	}
+	return true
+}
+
+func read_background_image(io_data *rc_io_data, path string) (func(io_data *rc_io_data) (bool, error), error) {
+	if is_index_arg(path) {
+		io_data.rc.Stream = false
+		return func(io_data *rc_io_data) (bool, error) {
+			set_payload_data(io_data, "index:"+path)
+			return true, nil
+		}, nil
+	}
+	return read_window_logo(io_data, path)
+}