Print warnings when reading write-only VERA register, or writing read-only

[irmen]

Box16's equivalent of X16Community/x16-emulator#359

Implemented after Wavicle's sleuthing for the cause of the Another World graphics glitch
see forum post https://cx16forum.com/forum/viewtopic.php?p=37485#p37485

example output

Warning: 086E read from write-only VERA register at 9F2B (DCSEL=6)
Warning: 0875 read from write-only VERA register at 9F2C (DCSEL=6)

question @indigodarkwolf I don't understand the difference between the "debug" versions of several routines and structures vs. the regular ones. Did I modify the right ones? When should you use the "debug" versions?
(example: debug_state6502.pc vs state6502.pc and vera_debug_video_read() vs vera_video_read())

[indigodarkwolf]

TL;DR: I think you got it right.

Most places would want to use state6502, because most places have always used that and the processor emulation didn't always support breakpoints that could trigger prior to an instruction running to completion. But you're doing something special.

debug_state6502 is the "debug copy" of the cpu's state made prior to executing the current operation, so that the CPU can return itself to that state in the event that an instruction halts mid-way due to a breakpoint. Since you're in the middle of an instruction, you probably want to refer to that debug copy since most folks won't easily be able to interpret a debug message that indicates a PC register that isn't aligned with the instruction, or worse, possibly points to the next instruction being executed. Lord knows I've certainly gotten confused by that in the past.