cogl-bitmap-pixbuf: Fix the rowstride used when copying a GdkPixbuf

In commit b780413e5a the GdkPixbuf loading code was changed so that
if it needs to copy the pixbuf then it would tightly pack it. However
it was still using the rowstride from the pixbuf so the image would
end up skewed. This fixes it to use the real rowstride.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2235

cogl/cogl-bitmap-pixbuf.c

@@ -278,7 +278,7 @@ _cogl_bitmap_from_file (const char   *filename,
                                      pixel_format,
                                      width,
                                      height,
-                                     rowstride,
+                                     n_channels * width,
                                      (CoglBitmapDestroyNotify) g_free,
                                      NULL);
 }