[gthumb] fixed image upscaling

[Paolo Bacchilega <paobac src gnome org>]

commit 51adcca929f6fc456eb18b655f78f9f7739b8c9f
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sat Apr 24 23:05:35 2021 +0200

    fixed image upscaling

 gthumb/cairo-scale.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
---
diff --git a/gthumb/cairo-scale.c b/gthumb/cairo-scale.c
index fab1c004..081ad8d8 100644
--- a/gthumb/cairo-scale.c
+++ b/gthumb/cairo-scale.c
@@ -351,6 +351,7 @@ horizontal_scale_transpose (cairo_surface_t *image,
                            ScaleReal        scale_factor,
                            resize_filter_t *resize_filter)
 {
+       ScaleReal  scale;
        ScaleReal  support;
        int        y;
        int        image_width;
@@ -365,9 +366,12 @@ horizontal_scale_transpose (cairo_surface_t *image,
        if (resize_filter->cancelled)
                return;
 
-       support = (1.0 / scale_factor) * resize_filter_get_support (resize_filter);
-       if (support < 0.5)
+       scale = MAX ((ScaleReal) 1.0 / scale_factor, 1.0);
+       support = scale * resize_filter_get_support (resize_filter);
+       if (support < 0.5) {
                support = 0.5;
+               scale = 1.0;
+       }
 
        image_width = cairo_image_surface_get_width (image);
        scaled_width = cairo_image_surface_get_width (scaled);
@@ -378,6 +382,7 @@ horizontal_scale_transpose (cairo_surface_t *image,
        dest_rowstride = cairo_image_surface_get_stride (scaled);
        weights = g_new (ScaleReal, 2.0 * support + 3.0);
 
+       scale = 1.0 / scale;
        for (y = 0; y < scaled_height; y++) {
                guchar    *p_src_row;
                guchar    *p_dest_pixel;
@@ -412,7 +417,7 @@ horizontal_scale_transpose (cairo_surface_t *image,
 
                density = 0.0;
                for (n = 0; n < stop - start; n++) {
-                       weights[n] = resize_filter_get_weight (resize_filter, scale_factor * ((ScaleReal) 
(start + n) - bisect + 0.5));
+                       weights[n] = resize_filter_get_weight (resize_filter, scale * ((ScaleReal) (start + 
n) - bisect + 0.5));
                        density += weights[n];
                }