backend: Allow overriding the Cogl drivers chain

We have an hardcoded list of drivers we have to go through when creating
a Cogl context. Some platforms may expose those drivers, but not be the
preferred ones.

In order to allow users and system integrators to override the list of
drivers, we should crib the same approach used by GDK, and have an
environment variable with a list of drivers to try.

The new environment variable is called `CLUTTER_DRIVER` and accepts a
comma-separated list of driver names, which will be tested in sequence
until one succeeds. There's also an additional '*' token which is used
to ask Clutter to fall back to the internally defined preferred list of
drivers.

https://bugzilla.gnome.org/show_bug.cgi?id=742678

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