applied changes from 8300e1e9a38c8d28e0710569d22be71f6c1abca9

applied changes from 8300e1e9a38c8d28e0710569d22be71f6c1abca9
             through 96183796df0c52bb91e9828948c24cfa36845c1d

Original commit message:
Start looking at the GError parameter every time we call these functions:
- gdk_pixbuf_loader_write
- gdk_pixbuf_loader_close
- gdk_pixbuf_new_from_file
- gdk_pixbuf_new_from_file_at_size
- gdk_pixbuf_new_from_file_at_scale

There are times when gdkpixbuf returns a semi-invalid GdkPixbuf object and
also sets the GError.  If this happens we want to discard and ignore the
GdkPixbuf object because it can cause problems.  For example, calling
gdk_pixbuf_scale_simple() causes gdkpixbuf to rapidly consume memory in
an infinite loop.  And that's bad.

This commit adds some helper functions to gtkutils.[c|h] that make it a
little easier to check the GError value.  We should use them everywhere
we call any of the above functions.

--HG--
branch : release-2.9.0
extra : convert_revision : 1b3d9b3df8497b2273716e872abfc59b48a567d4