(fn_jpeg_stdio_src): Don't define it.

(fn_jpeg_stdio_src): Don't define it.
(init_jpeg_functions): Don't initialize `fn_jpeg_stdio_src'.
(our_common_init_source): Rename from `our_init_source'.
(our_common_term_source): Rename from `our_term_source'.
(our_memory_fill_input_buffer): Rename from `our_fill_input_buffer'.
(our_memory_skip_input_data): Rename from `our_skip_input_data'.
(jpeg_memory_src): Use the new names.
(struct jpeg_stdio_mgr): New struct.
(JPEG_STDIO_BUFFER_SIZE): New constant.
(our_stdio_fill_input_buffer, our_stdio_skip_input_data, jpeg_file_src): New
functions.
(jpeg_load): Use `jpeg_file_src' instead of `fn_jpeg_stdio_src'.