According to OpenGL spec, a shader object will only be flagged
for deletion unless it has been detached; when a program object
is deleted, those shader objects attached to it will be detached
but not deleted unless they have already been flagged for deletion.
So we shall detach a shader object before it is deleted, and delete
it before the program object is deleted best.
The GtkGears widget is a bit too chatty, especially when used inside
demos like the fishbowl. Let's use g_debug() instead of g_print() for
the GL debugging message.
