Changes

&F995 : 0000YYYY : Input B, mouse Y

Buttons 1, 2, 3 are the same for joystick and mouse input A/B.

&F996 and &F997 ports are unused.

About usages:

- Reading from BASIC with 50Hz interrupt is fine to handle a fast char pointer with acceleration.

- Reading from ASM with 300Hz interrupt is fine to handle a fast pixel pointer with acceleration.