GIF: Introduce the GIF FIFO

* The GIF similarly to the VIF also contains a 16 qword FIFO
for queueing data. The implementation is very similar to the VIF
and I used similar function names to make it easier to follow.

* In addition introduce proper reset routines. Messing with the
this pointer is not possible in our case, because it breaks the
Handler infrastructure that relies on the pointer not changing.