Fix loading huge JPEGs

They fell back to gdk-pixbuf, then misrendered in the thumbnailer,
and crashed the program when loaded directly.

The second best we can do is scale them down, right after tiling,
which is a complex feature to add.

fiv-io.c

@@ -1112,6 +1112,28 @@ open_libjpeg_turbo(
 		? TJPF_CMYK
 		: (G_BYTE_ORDER == G_LITTLE_ENDIAN ? TJPF_BGRX : TJPF_XRGB);
 
+	// The limit of Cairo/pixman is 32767. but JPEG can go as high as 65535.
+	// Prevent Cairo from throwing an error, and make use of libjpeg's scaling.
+	// gdk-pixbuf circumvents this check, producing unrenderable surfaces.
+	const int max = 32767;
+
+	int nfs = 0;
+	tjscalingfactor *fs = tjGetScalingFactors(&nfs), f = {0, 1};
+	if (fs && (width > max || height > max)) {
+		for (int i = 0; i < nfs; i++) {
+			if (TJSCALED(width, fs[i]) <= max &&
+				TJSCALED(height, fs[i]) <= max &&
+				fs[i].num * f.denom > f.num * fs[i].denom)
+				f = fs[i];
+		}
+
+		add_warning(ctx,
+			"the image is too large, and had to be scaled by %d/%d",
+			f.num, f.denom);
+		width = TJSCALED(width, f);
+		height = TJSCALED(height, f);
+	}
+
 	cairo_surface_t *surface =
 		cairo_image_surface_create(CAIRO_FORMAT_RGB24, width, height);
 	cairo_status_t surface_status = cairo_surface_status(surface);

fiv-thumbnail.c

@@ -163,6 +163,15 @@ adjust_thumbnail(cairo_surface_t *thumbnail, double row_height)
 
 	cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
 	cairo_paint(cr);
+
+	// Note that this doesn't get triggered with oversize input surfaces,
+	// even though nothing will be rendered.
+	if (cairo_surface_status(thumbnail) != CAIRO_STATUS_SUCCESS ||
+		cairo_surface_status(scaled) != CAIRO_STATUS_SUCCESS ||
+		cairo_pattern_status(pattern) != CAIRO_STATUS_SUCCESS ||
+		cairo_status(cr) != CAIRO_STATUS_SUCCESS)
+		g_warning("thumbnail scaling failed");
+
 	cairo_destroy(cr);
 	return scaled;
 }