Bug 781020 - GIMP UI vector icons are drawn way too small.

Though the bug was mostly fixed, it seems to still happen on Windows XP,
where apparently no content type had been registered for SVG.
GTK+ developers don't seem too keen to patch GTK+ 2.24 for a platform
which they don't support anymore.

Also if not mistaken, GIMP does not officially support Windows XP
anymore either. A patch though has already been provided by Edward E.
and it really doesn't look like it could break anything since it just
adds a last "else if" when everything else failed (and inside a #ifdef
affecting only WIN32 builds).
So let's just add it in our builds at least. We still don't have support
for it, but I see no reason to just refuse a minor patch which won't
break anything else.

build/windows/patches/gtk+-2.24-bug781020.patch

@@ -0,0 +1,32 @@
+From b14a762e9940739df52c81e6bc888cc01579ed46 Mon Sep 17 00:00:00 2001
+From: Edward E <develinthedetail@gmail.com>
+Date: Fri, 1 Dec 2017 12:32:28 -0600
+Subject: [PATCH] icontheme (win32): detect SVG files by extension if
+ unregistered
+
+It seems XP shipped without a registered Content Type for .svg
+
+https://bugzilla.gnome.org/show_bug.cgi?id=781020
+---
+ gtk/gtkicontheme.c | 5 +++++
+ 1 file changed, 5 insertions(+)
+
+diff --git a/gtk/gtkicontheme.c b/gtk/gtkicontheme.c
+index 532d94b..98cb5f2 100644
+--- a/gtk/gtkicontheme.c
++++ b/gtk/gtkicontheme.c
+@@ -2927,6 +2927,11 @@ icon_info_ensure_scale_and_pixbuf (GtkIconInfo  *icon_info,
+ 
+               if (mime_type && strcmp (mime_type, "image/svg+xml") == 0)
+                 is_svg = TRUE;
++#ifdef G_OS_WIN32
++              else
++                if (mime_type && strcmp (mime_type, "application/x-ext-svg") == 0)
++                  is_svg = TRUE;
++#endif
+             }
+ 
+           g_object_unref (file_info);
+-- 
+2.9.1
+