plug-ins: we still depends on librsvg 2.40.x.

In commits a6756b515b and 23da44e3c8, newer API of librsvg were used, over our
current minimum requirement. The reason why we don't bump this requirement is
that librsvg 2.40.x are the latest versions which are full-C. While we have
nothing against Rust, it is not available on all existing platforms, and we
really want to have GIMP buildable everywhere, while also have SVG support
always present (even though it's a vector format, it's an important one in
graphics work nowadays).
See issue #6821.

So in the end, let's keep the old minimum requirement, but test against librsvg
version to compiler either with old API or new one. The changes are limited
enough that it's not too much a bother.

plug-ins/common/file-svg.c

@@ -392,6 +392,68 @@ load_image (GFile            *file,
   return image;
 }
 
+#if ! LIBRSVG_CHECK_VERSION(2, 46, 0)
+/* XXX Why we keep old deprecated implementation next to newer librsvg API is
+ * because librsvg uses Rust since version 2.41.0. There are 2 raised problems:
+ * 1. Rust is still not available or well tested on every architecture/OS out
+ *    there yet so unless we decide to have SVG support as optional again, it
+ *    would make GIMP non-available on these.
+ * 2. There are some technical-ideological position against Rust for bundling
+ *    and linking dependencies statically.
+ * While the second point may or may not be a problem we want to take into
+ * account (I guess that it mostly depends on the amount of additional
+ * maintenance work it would imply), the first is definitely enough of a reason
+ * to keep an old version requirement.
+ * See also report #6821.
+ */
+static void
+load_set_size_callback (gint     *width,
+                        gint     *height,
+                        gpointer  data)
+{
+  SvgLoadVals *vals = data;
+
+  if (*width < 1 || *height < 1)
+    {
+      *width  = SVG_DEFAULT_SIZE;
+      *height = SVG_DEFAULT_SIZE;
+    }
+
+  if (!vals->width || !vals->height)
+    return;
+
+  /*  either both arguments negative or none  */
+  if ((vals->width * vals->height) < 0)
+    return;
+
+  if (vals->width > 0)
+    {
+      *width  = vals->width;
+      *height = vals->height;
+    }
+  else
+    {
+      gdouble w      = *width;
+      gdouble h      = *height;
+      gdouble aspect = (gdouble) vals->width / (gdouble) vals->height;
+
+      if (aspect > (w / h))
+        {
+          *height = abs (vals->height);
+          *width  = (gdouble) abs (vals->width) * (w / h) + 0.5;
+        }
+      else
+        {
+          *width  = abs (vals->width);
+          *height = (gdouble) abs (vals->height) / (w / h) + 0.5;
+        }
+
+      vals->width  = *width;
+      vals->height = *height;
+    }
+}
+#endif /* ! LIBRSVG_CHECK_VERSION(2, 46, 0) */
+
 /*  This function renders a pixbuf from an SVG file according to vals.  */
 static GdkPixbuf *
 load_rsvg_pixbuf (GFile            *file,
@@ -401,12 +463,15 @@ load_rsvg_pixbuf (GFile            *file,
                   GError          **error)
 {
   GdkPixbuf  *pixbuf  = NULL;
-  cairo_surface_t *surf = NULL;
-  cairo_t *cr = NULL;
   RsvgHandle *handle;
-  RsvgRectangle viewport = { 0, };
-  guchar *src;
-  gint y;
+
+#if LIBRSVG_CHECK_VERSION(2, 46, 0)
+  cairo_surface_t *surf = NULL;
+  cairo_t         *cr = NULL;
+  RsvgRectangle    viewport = { 0, };
+  guchar          *src;
+  gint             y;
+#endif
 
   handle = rsvg_handle_new_from_gfile_sync (file, rsvg_flags, NULL, error);
 
@@ -421,6 +486,7 @@ load_rsvg_pixbuf (GFile            *file,
     }
 
   rsvg_handle_set_dpi (handle, vals->resolution);
+#if LIBRSVG_CHECK_VERSION(2, 46, 0)
   pixbuf = gdk_pixbuf_new (GDK_COLORSPACE_RGB, TRUE, 8,
                            vals->width, vals->height);
   surf = cairo_image_surface_create_for_data (gdk_pixbuf_get_pixels (pixbuf),
@@ -437,7 +503,6 @@ load_rsvg_pixbuf (GFile            *file,
 
   cairo_destroy (cr);
   cairo_surface_destroy (surf);
-  g_object_unref (handle);
 
   /* un-premultiply the data */
   src = gdk_pixbuf_get_pixels (pixbuf);
@@ -455,6 +520,12 @@ load_rsvg_pixbuf (GFile            *file,
 
       src += gdk_pixbuf_get_rowstride (pixbuf);
     }
+#else
+  rsvg_handle_set_size_callback (handle, load_set_size_callback, vals, NULL);
+  pixbuf = rsvg_handle_get_pixbuf (handle);
+#endif
+
+  g_object_unref (handle);
 
   return pixbuf;
 }
@@ -471,6 +542,9 @@ load_rsvg_size (GFile            *file,
   RsvgHandle        *handle;
   gboolean           has_size;
   gdouble            width, height;
+#if ! LIBRSVG_CHECK_VERSION(2, 52, 0)
+  RsvgDimensionData  dim;
+#endif
 
   handle = rsvg_handle_new_from_gfile_sync (file, rsvg_flags, NULL, error);
 
@@ -479,7 +553,15 @@ load_rsvg_size (GFile            *file,
 
   rsvg_handle_set_dpi (handle, vals->resolution);
 
-  if (rsvg_handle_get_intrinsic_size_in_pixels (handle, &width, &height))
+#if LIBRSVG_CHECK_VERSION(2, 52, 0)
+  rsvg_handle_get_intrinsic_size_in_pixels (handle, &width, &height);
+#else
+  rsvg_handle_get_dimensions (handle, &dim);
+  width  = (gdouble) dim.width;
+  height = (gdouble) dim.height;
+#endif
+
+  if (width > 0.0 && height > 0.0)
     {
       vals->width  = ceil (width);
       vals->height = ceil (height);