Make testsuite fail if we lack pixbuf loaders

Add a test that requires that we have png and jpeg
loaders.

testsuite/gdk/meson.build

@@ -11,6 +11,7 @@ tests = [
   'encoding',
   'keysyms',
   'memorytexture',
+  'pixbuf',
   'rectangle',
   'rgba',
   'seat',

testsuite/gdk/pixbuf.c

@@ -0,0 +1,31 @@
+#include <gdk-pixbuf/gdk-pixbuf.h>
+
+int
+main (int argc, char *argv[])
+{
+  GSList *formats;
+  gboolean have_png, have_jpeg;
+
+  have_png = FALSE;
+  have_jpeg = FALSE;
+
+  formats = gdk_pixbuf_get_formats ();
+
+  for (GSList *l = formats; l; l = l->next)
+    {
+      GdkPixbufFormat *format = l->data;
+      const char *name;
+
+      name = gdk_pixbuf_format_get_name (format);
+
+      if (strcmp (name, "png") == 0)
+        have_png = TRUE;
+      else if (strcmp (name, "jpeg") == 0)
+        have_jpeg = TRUE;
+    }
+
+  if (!have_png || !have_jpeg)
+    return 1;
+
+  return 0;
+}