Fix invalid unreference of CoglTexture

cogl_texture_get_data doesn't increase the refcount, so don't try
to decrement it.

Signed-off-by: Colin Walters <walters@verbum.org>

https://bugzilla.gnome.org/show_bug.cgi?id=608512

src/shell-drawing.c

@@ -84,7 +84,6 @@ shell_fade_app_icon (ClutterTexture *source)
 
   cogl_texture_get_data (texture, COGL_PIXEL_FORMAT_RGBA_8888_PRE,
                          rowstride, pixels);
-  cogl_texture_unref (texture);
   
   fade_start = width / 2;
   fade_range = width - fade_start;