CGA status register

The CGA status register (io port 0x03DA) should be implemented correctly.

Many demos check the status of the following bits to sync screen updates:

    Bit 0 - horizontal retrace in progress
    Bit 3 - in vertical retrace

Currently, we hack around this by changing the bits over time in https://github.com/martinlindhe/dustbox-rs/blob/master/src/cpu.rs#L3586