Only initialise PS/2 mouse port after mouse enable command

[stevesims]

Currently the PS/2 mouse port is automatically set up on startup, but the VDP starts in a state where the mouse is not "enabled".

There is a cost involved at boot time for this approach, making the boot sequence take longer.

It is suggested that the PS/2 mouse port setup could be conducted the first time the mouse enable command VDU 23,0,&89,0 is executed. This would make the boot time faster.

[stevesims]

on experimenting around this I found there is a bug somewhere inside vdp-gl that prevents the fabgl::PS2Controller::begin() function from being called twice. changing the agon-vdp code to only set up the keyboard port of course works, but the subsequent call to re-start the PS2 controller with both ports enabled results in a crash when initialising the mouse

for now, this therefore prevents this idea from being implemented - the underlying bug in vdp-gl needs to be fixed first