gtk-utils.c: Use cairo in show_image() rather than GdkPixbuf

gtk-utils.c: Use cairo in show_image() rather than GdkPixbuf

GdkPixbufs are not premultiplied, so when using them to display pixman
images, there is some unecessary conversions going on: First the image
is converted to non-premultiplied, and then GdkPixbuf premultiplies
before sending the result to the X server. These conversions may cause
the displayed image to not be exactly identical to the original.

This patch just uses a cairo image surface instead, which avoids these
conversions.

Also make the comment about sRGB a little more concise.