Patch from davefiddes. Original commit message :
The bank number is stored in a pointer value but is used exclusively as a
bank number integer reference. To do this safely it needs to be an intptr_t.
As part of this, added emulation of the onboard AVGA2, and fixed a couple of
bugs in the Cirrus Logic extended write modes.
Currently chipset (VLSI SCAMP) is missing EMS emulation.
