COGL_DEBUG=all wasn't previously useful as there are several options
that change the behaviour of Cogl and all together wouldn't help anyone
debug anything.

This patch makes it so COGL_DEBUG=all|verbose now only enables options
that don't change the behaviour of Cogl, i.e. they only affect the
amount of noise we'll print to a terminal.

In addition to that this patch also improves the output from
COGL_DEBUG=help so we now print a table of options including one liner
descriptions of what each option enables.