Move image save operation off the main thread

[kleisauke]

Required for libvips/libvips#5092.

The revised save_options_eval() is similar to:

vipsdisp/src/imagewindow.c

Lines 453 to 523 in e387036

	typedef struct _EvalUpdate {
	Imagewindow *win;
	int eta;
	int percent;
	} EvalUpdate;
	static gboolean
	imagewindow_eval_idle(void *user_data)
	{
	EvalUpdate *update = (EvalUpdate *) user_data;
	Imagewindow *win = update->win;
	char str[256];
	VipsBuf buf = VIPS_BUF_STATIC(str);
	vips_buf_appendf(&buf, "%d%% complete, %d seconds to go",
	update->percent, update->eta);
	gtk_progress_bar_set_text(GTK_PROGRESS_BAR(win->progress),
	vips_buf_all(&buf));
	gtk_progress_bar_set_fraction(GTK_PROGRESS_BAR(win->progress),
	update->percent / 100.0);
	g_object_unref(win);
	g_free(update);
	return FALSE;
	}
	static void
	imagewindow_eval(VipsImage *image,
	VipsProgress *progress, Imagewindow *win)
	{
	double time_now;
	EvalUpdate *update;
	/* We can be ^Q'd during load. This is NULLed in _dispose.
	*/
	if (!win->progress_timer)
	return;
	time_now = g_timer_elapsed(win->progress_timer, NULL);
	/* Throttle to 10Hz.
	*/
	if (time_now - win->last_progress_time < 0.1)
	return;
	win->last_progress_time = time_now;
	#ifdef DEBUG_VERBOSE
	printf("imagewindow_eval: %d%%\n", progress->percent);
	#endif /*DEBUG_VERBOSE*/
	/* This can come from the background load thread, so we can't update
	* the UI directly.
	*/
	update = g_new(EvalUpdate, 1);
	update->win = win;
	update->percent = progress->percent;
	update->eta = progress->eta;
	/* We don't want win to vanish before we process this update. The
	* matching unref is in the handler above.
	*/
	g_object_ref(win);
	g_idle_add(imagewindow_eval_idle, update);
	}

[kleisauke]

FWIW, I think we should also consider re-syncing vipsdisp with nip4. For example, I noticed that we could cherry-pick commit libvips/nip4@b546407.

vipsdisp/src/infobar.c

Lines 225 to 238 in e387036

	/* Block outside the image.
	*/
	if (update->image_x >= 0 &&
	update->image_y >= 0 &&
	update->image_x < image->Xsize &&
	update->image_y < image->Ysize)
	/* Fetch from image, even though this can be very slow.
	* This is run in a bg thread, so speed should not matter too much.
	*/
	update->result = !vips_getpoint(image,
	&update->vector, &update->n,
	update->image_x, update->image_y,
	"unpack_complex", TRUE,
	NULL);