We use polymorphic exception objects, so catching by value invokes
the copy constructor and stuff that we don't really want.

This will allow us to use better formats that preserve the entire
alpha channel.

This is based on comparing CPU time with elapsed time, which means
that the tests can be heavily influenced by other load on the test
machine.

We need more callbacks for correct operation of multi-threaded
rect decoding.

Done in preparation for multi-core decoding. Keeps the complexity
out of the other classes. This also moves ownership of the
framebuffer in to CConnection. It's the CConnection object that is
aware of the threads and how to synchronise with them. Therefore
the ownership of the framebuffer must also be there to make sure
it isn't deleted whilst threads are working.

This prevents unhandled exceptions if the file isn't found.