[genesisplusgx] "VDP mode" option (for eliminate region lockups / megakey behavior)

[eugene-s-nesdev]

Hello, bizhawk-devs!
Can you add "VDP mode" option to genesisplusgx settings please?
This option is already implemented in ekeeke's Wii/Gamecube genplusgx port and the libretro port also.

Information:
ekeeke/Genesis-Plus-GX#558
libretro/Genesis-Plus-GX#349

There are many of good NTSC games that haven't been released for the PAL region,
and now it works very good on PAL mode using this genplusgx feature (or using megakey, if we're talking about hardware)
Also, some of NTSC games have slightly different behavior compared to their European releases, so this feature could be useful.

[YoshiRulz]

config.region_detect is already exposed (as "Autodetect Region", bit of a misnomer), config.vdp_mode is currently hardcoded (and was before the update):

BizHawk/waterbox/gpgx/cinterface/cinterface.c

Line 921 in 0122dec

config.vdp_mode = 0; /* = AUTO (1 = NTSC, 2 = PAL) */

[eugene-s-nesdev]

Autodetect region - is correct, it will prevent region lockups.
Ekeeke / libretro devs have added an additional option to force VDP mode (0 = auto, 1 = 60Hz, 2 = 50Hz)
and user can set it regardless to region detect.

According to ekeeke's README doc:
https://github.com/ekeeke/Genesis-Plus-GX/blob/master/gx/docs/README.pdf

Region
let you force the region setting for the emulated console:
• AUTO: region is automatically detected, through ROM header and internal game database.
• PAL: forces PAL region code
• USA: forces NTSC-U region code
• JAPAN: forces NTSC-J region code
(Some games might act differently depending on the selected region setting but also may not run correctly or display an error message on startup if they are region protected.)

VDP Mode
let you force emulated hardware TV mode, independently from console region:
• AUTO: 50/60hz mode is automatically detected from console region
• NTSC: forces NTSC mode (60hz)
• PAL: forces PAL mode (50hz)
(This option can help make region-protected games running at a different speed without changing the hardware region.)