commit 90dab77f5639ee9d3d75eb739557350aa432e353 changed the target surface format from always ARGB32 to depend upon whether the source surface has an alpha channel. This fails to take into account the result of the color transformation which may require the alpha channel.

The simple fix would be:
diff --git a/swfdec/swfdec_renderer.c b/swfdec/swfdec_renderer.c
index 787e592..dcf74dd 100644
--- a/swfdec/swfdec_renderer.c
+++ b/swfdec/swfdec_renderer.c
@@ -424,11 +424,10 @@ swfdec_renderer_transform (SwfdecRenderer *renderer, cairo_surface_t *surface,
 
   /* FIXME: This function should likely be a vfunc. 
    * Or better: it should compile to a shader */
+  target = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, rect->width, rect->height);
   if (cairo_surface_get_content (surface) & CAIRO_CONTENT_ALPHA) {
-    target = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, rect->width, rect->height);
     mask = 0;
   } else {
-    target = cairo_image_surface_create (CAIRO_FORMAT_RGB24, rect->width, rect->height);
     mask = SWFDEC_COLOR_COMBINE (0, 0, 0, 0xFF);
   }
   cairo_surface_set_device_offset (target, -rect->x, -rect->y);